STUCK PIPE PREVENTION WHILE DRILLING WELLS to LIMESTONE & SANDSTONE RESERVOIRS – SOUTHERN IRAQ

T. Eren M. H. Azeez B. A. Tawfeeq Batman University South Oil Company South Oil Company Petroleum and Natural Gas Engineering Department, Batman Turkey Basra Iraq Basra Iraq tuna.eren@batman.edu.tr elhussainy2001@gmail.com ABSTRACT Stuck pipe occurrences are observed to be a common trouble during the increased drilling operations in the Southern Iraq Oilfields. At the time of the preparation of this paper there were more than 12 rigs actively working in the Field. This paper is giving the information about the case studies encountered while drilling wells in an Oilfield in Southern Iraq. Examples of differential and mechanical stuck pipe occurrences are detailed; providing all necessary information to understand whether how the stuck pipes had occurred, and how the problem had been overcome if successful. Most of the stuckpipe occurrences are attributed to the sloughing and high permeability shale formations, overpressured drilling fluid hydrostatic and depleted reservoirs. As per industry literature it is widely known that recovering from a stuck pipe situation is time consuming, costly, with the high risk of losing the wellbore itself if not to re-drill it from the beginning. Damages occurring during the drilling course due to the stuckpipe occurrences may also create a poor quality well that is not up to the standard that it has been designed in the first place. In this study a number of stuck pipe occurrences are given and analyzed as per why the workstring got stuck. It is observed that the most important aspect in order to prevent the occurrence of stuck pipes is considering how situations as such could happen and make sure the step-wise drilling practices in effect are prepared appropriately that stuck occurrences are prevented and made possible to be dealt with. The golden rule is to make the planning and dealing with stuck pipe occurrences in the simplest way possible. INTRODUCTION The main objective of drilling oil, gas or a geothermal well is to ensure that a safe and economically cheapest well is drilled at the earliest. A bore hole must be large enough for the casing to pass freely with the little chance of getting stuck [1]. Troubles those of which occur in the course of drilling activities are mostly due to the times based on stuck pipe problems if not a well control operations [2]. In drilling industry the stuck pipe events are still the main reason of Non Productive Time (NPT) even though they are largely preventable by effective planning and following practices. The key is to read the downhole conditions and understand early indicative signs. The role of driller is at utmost importance for the implementations of the trouble free drilling, [3]. This paper is analyzing the stuck pipe events encountered during the development drilling campaign from the Southern Fields in Iraq. Complications related to stuck pipe can account for nearly half of total well cost, making stuck pipe one of the most expensive problems that can occur during a drilling operation. Stuck pipe often is associated with well control and lost circulation events; the two other costly disruptions to drilling operations and is a significant risk in high-angle and horizontal wells [4]. The consequences of a stuck pipe event not only is related with the elevated drilling costs, but also with the time lost due to not being able to produce the hydrocarbons within the projected time frame. The stuck pipe events given in this publication are the stuck pipes encountered in drilling vertically planned wells. The subsurface formations of the oil and gas field in Southern Iraq are consisted of sloughing shales, and high permeability formations drilling through which is required with relatively dense drilling fluids; preparing the recipe for the stuck pipe events. The brown fields in the area are also with reservoir pays under depleted states. In some Fields the depletion pressure is almost 35% less than the reference reservoir pressures at the time of the preparation of this document. CALCULATIONS FOR DIFFERENTIAL PRESSURE AND HOLE CLEANING Drilling in the depleted zones, where usually the pressure in the hole exceeds the pressure within the formation, may cause the drillstring to be stuck on the borehole and be embedded in the filter cake deposited across the well. The force to pull the pipe at some occasions could get extremely high. Figure 1 gives the drawing of the differential sticking mechanism. Mitchell [5] defined the projected width of the contact area is given in Equation 1 as follows: (1) whereas the differential pressure is calculated using Equation 2, (2) Figure 1. Differential sticking drawing. The major aims of an effective hydraulics program are: to clean the cuttings from the hole effectively, and to use most of the hydraulics power available to drill the hole. Failure of cleaning the wellbore when using conventional drilling fluids could result in accumulation of the cuttings in the annulus of the workstring and the wellbore, which may result in occurrence of a stuck pipe incident. Ozbayoglu et al. introduced a model for minimum liquid- and gas-flow rates which can be identified for having an acceptable cuttings concentration inside the wellbore as well as a preferably low frictional pressure drop for gasified drilling fluids [6], [7]. Annular velocity (AV) is one of the most important factors to achieve a feasible hole cleaning in vertical wells. The velocity of the drilling fluid shall exceed the velocity of the cuttings slipping back to the bottom of the wellbore due to the force of gravity pulling the cuttings. One rule of thumb on finding the right AV for vertical wells is as follows: “the AV has to be at least twice the slip velocity of the cuttings.” The slip velocity for natural settling of solid particles under laminar conditions is defined in Equation 3: V_s=(2gd_c 2(ρ_s-ρ_m ))/92.6μ (3) A field application method for estimating the minimum annular velocity to ensure appropriate wellbore cleaning is being fulfilled is based on Fullerton’s approach, [8]. This method assumes that the diameter of the cuttings is 0.25 in, the cuttings density is 2.5 sg and that the annular fluid velocity should be not less than twice the cuttings settling velocity. The approach is based on the density of the drilling fluid, i.e.: if df≤ 12 ppg (or 1.43 sg) then the minimum annular velocity is calculated using Equation 4: V_min=613/(d_h×ρ_df ) (4) If df > 12 ppg (or 1.43 sg) then use of the chart given in Figure 2 is suggested. Most of the stuck pipe problems occurred in the Southern Iraq Oil Fields occurred due to marl and sloughing shales, together with excessive drilling fluid density in borehole which resulted in stuck pipe incidents. The representative PPFG chart belonging to the Southern Oil Fields in Iraq is given in Appendix-A. Accurate PPFG charts are very important for the planning requirements of the wells and interventions in case of stuck pipe incidents. The drilling parameters acquired through mud logging units are very useful when analyzing the stuck pipe events not only after the pipe has stuck, but also an indication of a stuck pipe event is encountered. In this paper the parameters being given are acquired through the mud logging units. The data from specific equipment/machinery is transmitted after being measured by means of sophisticated transducers/sensors [9]. Figure 2. Flow rate vs hole size for minimum annular velocity if df > 12 ppg. The Southern Oilfields of Iraq consists of lithologies formed of clay and plastic marl formations with high reservoir permeability in which stuck pipe incidents may be encountered. The depleted reservoir formations may result in encountering circulation losses, whereas the naturally fractured vuggy dolomite texture limestone formations may cause complete losses while the drilling campaigns. The stuck pipe events which led to the sidetrack operations in given wells located in Southern Iraq are analyzed in this paper. All of the remedial attempts such as pumping acid and lubricant materials to free the stuck pipes were unsuccessful. At the time of the occurrence of stuck pipe events the use of diesel plugs were not allowed as per regulations in the scope of environmental concerns. LITERATURE SURVEY In literature the stuck pipe is defined as the situation in which drillpipe, collars, casing or tubing which cannot be pulled free from the wellbore. The wellbore diameter shall be large in size to accommodate the casing string designed to be set in place. Most of the time drillstrings shall be designed to have a margin of overpull magnitude which may be in the range of 50,000-100,000 lbs to account for the extra pull force in case of stuck pipe events [10], [11]. The stuck pipe events are preventable if the filter cake of the mud is thin and impermeable, filtercakes otherwise could be damaging in addition to the sticky behaviour which may cause differential stuck pipe events. Other two main groups of stuck pipe events are Solids Induced Packs Offs and Wellbore Geometry Related stuck pipe events. Figure 3 gives the mechanisms of pipe sticking. Figure 3. The mechanisms of stuck pipe occurrences. Devereux [12] stated that most of the cases of stuck pipe events; i.e. over 90%, are avoidable with good planning and listening to the hole. The most important factors in the prevention of stuck pipe occurrences are the training and crew awareness. Azar and Robello [13] mentioned that use of a top drive system would give the ability of reaming and back reaming in full stands while tripping in or out which minimizes the potential occurrence of stuck pipe events. Hopkins and Leicksenring [14] stated that real-time drilling data management systems could reduce stuck pipe occurrences by continuous monitoring of all the important drilling data trends via mudlogger’s computer. They suggested that the torque and drag trends of the hole will indicate for a possible stuck event, and the hole will provide more reliable information if the parameters are registered in a status of non-rotating string and no pumps on. The literature suggests that if a hole is drilled in the direction of the maximum horizontal stress then hole instability is minimized. Explaining this in other words would mean that; where the overburden stress is greater than the horizontal stress then a vertical borehole is more stable than a horizontal one. Aadnoy et al. [15] presented a mechanistic analysis of differentially stuck pipe in deviated wells. They derived equations to estimate the depth of the stuck point in deviated wellbores based on pull and rotation data register. The differential pressure across the stuck interval is the dominating factor, which stated as to be reduced, which can be done by displacing the well with a lighter fluid. Niznik et al. [16] stated that concentration if would be high enough while drilling then pack-offs and potential stuck pipes can occur in the Qatar's North Field. They mentioned that drilling an entire section length with total losses had not been attempted due to the higher risk of mechanically sticking the drill string from cuttings fallout and the complications imposed on well control management. Bradley et al. [17] stated that establishing a task force could reduce the costs associated with stuck pipe events up to 70%. Influence of drilling crew was found to be the most important factor as compared to introduction of new technologies. Proper handover practice whilst the shift changes were one of the significant reasons of the stuck pipe incidents among the outcomes of the study performed, which also resulted to give the importance of using top drive systems in reducing the times to reduce freeing the stuck pipes. Yarim et al. [18] observed that the majority of the training material available in the industry is providing information on how to free the stuck pipe, rather than on how to prevent the occurrence of the incidents. Guzman et al. [19] found that most of the hole-cleaning stuck pipe incidents were directly related to getting stuck while pumping out of the hole and while back reaming tight spots. They stated that when the annulus is loaded with cuttings and the string attempted to be pulled upwards the risk of getting stuck is higher. It was suggested that the project-specific stuck pipe risks and mitigation measures are distributed and updated as the project is executed and risks changed. The innovative approaches suggested in their study resulted in a 37% reduction of stuck-pipe occurrences during a year in comparison to the previous year. The wells which had the stuck in pipe penetrated formations of marl and limestone with impeded sloughing shale or layers of sloughing shale formations. However in the early stages of the development campaign it noted that the stuck pipe incidents were inevitable due to lack of field knowledge and competent field staff. STUCK PIPE OCCURENCES Some stuck pipe events occurred in the Southern Iraq Oil Fields is summarized together with the BHA, and drilling parameter details. The Lithology of the Southern Iraq Oil Fields are similar to that of the given in Appendix-B, after Al-Saeedi et al. [20] in which the stratigraphy/lithology of Kuwait is highlighted giving the troublesome formations. The main drilling problems being encountered in the area are not limited to the Major Loss Zones in Dammam and Hartha Formations, Sulphurous water from Tayarat/Radhuma Formations, and Wellbore Instability problems in Burgan Formation. Well-A Stuck Pipe Incident The first stuck pipe event is given for Well-A. The drilling was continuing in 12 ¼ in hole section at the depth of 2235 m, with the following parameters: Q = 3200 lpm, SPP = 3104 psi, MW = 1.15 sg. Observed partial losses at this depth, decreased Q step by step from 3300 lpm to 2500 lpm, drilled to 2247 m, lost 285 bbl mud. Continued drilling with 2000 lpm down to the bottom of Mishrif (interval: 2214 – 2378 m). Increased the Q to 2400 lpm and drilled ahead to 2453 m, SPP = 2000 psi, the lost rate ranged between 30 and 50 bbl/h. Circulated the hole and spot 130 bbl LCM pill. POOH to 2407 m. Attempted to pump out, however SPP increased to 2500 psi. POOH with 40 tons over pull up to 2265 m. Jarred up and down without success. Lost the flow while back reaming. Workstring stuck at 2265 m. Figure 4 shows the drilling parameters and sketch of the BHA at the moment of drilling at 2453 m. The Q indicates that the Q was reduced at the depth of 2235 m, the depth at which losses occurred. The Mishrif Formation is in depletion and loss circulations are expected. It is observed that at the moment of drilling prior to the stuck pipe event; Q was at a magnitude that is approximately equal to providing minimum annular velocity. It is important to remember that the 19 hours drilling process (between the 2324-2453 m interval) continued with partial losses, 20-90 bbl/h. The losses resulted in reduction in the Q across the Mishrif, even though the HI-VIS pills and LCM pills were being pumped at regular interval, the accumulation of cuttings in the annulus was not able to be prevented. Following the Fullerton’s approach the minimum flow rate is calculated to be approximately 1940 lpm if the hole diameter was 12 ¼ in in diameter, dcuttings = 0.25 in, MW = 1.15 sg, whereas the Q through the pump was as low as approximately 2000 lpm. If the actual hole diameter is taken to be non-cylindrical; approximately 13.5 in in diameter, the minimum flow rate would then be approximately 2200 lpm. The drilling continued across formations consisting of limestone impeded with shale layers. Figure 4. WELL-A Stuckpipe information. The calculations reveal that the insufficient flow rate accompanied with relatively high rate of penetration; resulted in accumulation of excessive cuttings in the annulus, and the stuck pipe event is observed to have been expected. Even though the bottom up time was 70 mins, and the hole was circulated for two bottoms up, and whenever the string pulled out, the stuck pipe event occurred at the moment the top of the upper stabilizer came in contact with the accumulation of cuttings across the Mishrif Formation. The string was pulled to 2294 m by 40 ton overpull, beyond the 2268 m depth the overpull attempts were unsuccessful. The circulation was also lost due to the accumulated cuttings and LCM inside the string and annulus. The stuck pipe incident occurred was reported to be a solids related pack-off with inadequate hole cleaning. The well was sidetracked by means of a back off at 2168 m; following all of the fishing attempts, including acid pumping. The stuck pipe event that happened in Well-A would have been prevented should more attention had been put in place. Proper hole cleaning practices must have been applied and minimum annular flow rate must have been attained. The circulation prior to the pull out must have been kept longer. Well-B-ST-1 Stuck Pipe Incident The second stuck pipe event is given for Well-B. Figure 5 shows the drilling parameters and sketch of the BHA at the moment of drilling at 3276 m. The ROP is observed to have increased following a change in the major drilling parameters at the depth of 3200 m. While drilling 12 ¼ in hole section at 3276 m, the drilling stopped, and without any circulation at the end of almost 60 hours of continuous drilling from 3180 m; the string was pulled out to 3245 m, the tight spot was over pulled 20 tons. Back reamed to 3139 m. Serviced the top drive for approximately one hour. RIH to 3167 m, slacked off 15 tons and reamed to 3276 m. Circulated for one hour and half. Pumped out to 3249 m, and back reamed to 2943 m. Attempted to circulate, however annulus packed off, pipe stuck. Figure 5. WELL-B Stuckpipe-1 information. As contrary to the case explained in Well-A, in Well-B the drilling parameters especially the flow rate was rather constant. The stuck pipe had taken place following the long duration of drilling without any pumping of HI-VIS pill or adequate circulation. The annulus has been filled in with cuttings, and the string having been stopped in the open hole, maintenance of the rig machinery has been performed. The circulation that has been performed for duration of 1.5 hours is observed not to be enough considering the fact that the hole is from a practical point of view not in gauge. The circulation should have been kept longer for at least two bottoms up as per best petroleum industry practices and experience in the Oil Fields of Sothern Iraq. The stuck pipe incident occurred was reported to be a solids related pack-off with inadequate hole cleaning. The stuck pipe event that happened in Well-B-ST-1 would have also been prevented should more attention had been put in place and maintenance of the rig machinery not performed while the BHA was across the open hole interval. Well-B-ST-2 Stuck Pipe Incident The second stuck pipe occurred in Well-B took place along the deeper sections of the well; after having cored from 3749 m to 3777 m in 15 hours. Figure 6 shows the drilling parameters and sketch of the BHA at the moment of coring at 3777 m. When attempted to pump out of hole, after having circulated for two hours only, noted that the string was differentially stuck at 3775 m. The circulation was performed with a flow rate of 880 lpm. However the Fullerton’s approach indicates that the minimum flow rate must have been at a rate of 1020 lpm, which reveals that the hole cleaning was not at the desired rate. The drilling fluid density was reduced from 1.77 sg to 1.74 sg to have less differential pressure across the stuck interval. Attempts to release the string by pulling and slacking off 60 tons did not provide enough pull to free the string. The stuck pipe incident occurred was reported to be a differential sticking in kind with permeable zone exposed to high overbalance. Successive attempts of pumping fluid spots also proved to be unsuccessful. Backed off the string and continued operations performing a sidetrack. Figure 6. WELL-B Stuckpipe-2 information. The grade of the DP limited the maximum overpull of 100 tons only. The free point indication tools revealed that the bottom hole assembly was stuck for a length of approximately 100 m, which indicated that the freeing pull force is high above the limits of the workstring. RECOMMENDATIONS The string shall be kept in motion whenever applicable at all times if hole cleaning is calculated to be a problem. The loss zone shall be determined at the earliest convenience and to the accurate level possible for correct mitigation action. LCM particles of mean size greater than 1/3 size of the smallest bit nozzles shall not be used when curing the losses. Rule of thumb states that the LCM concentrations greater than 50 ppb for losses upto 50 bbl/h shall be pumped. Denser concentrations may not be effective and though result in having excessive filter cakes. It is observed that correctly sized LCM concentrations work efficiently. In case the flow rate is reduced due to the loss circulation, the hole may become dirtier, for this reason elongated periods of circulation may be established. Increasing the yield point and low shear rate viscosity may assist hole cleaning. Properties of the drilling fluid must be closely monitored; the API fluid loss must be kept as low as possible to prevent the excessive filter cake creation across the permeable levels. The following best practices are the recommended to prevent occurrence of stuck pipes while drilling in the Southern Fields of Iraq: Periodical High Viscosity pills are suggested to be pumped at regular intervals (e.g. 15 m or every two hours) to ensure efficient hole cleaning. The pills will be incorporated into the mud system and eventually build up a good rheology. The drilling fluid rheology may be supplemented with addition of polymer to maintain effective hole-cleaning. While across open hole never keep the workstring stationary, always ensure the string is reciprocating, rotating and circulation in progress if drilling is not continuing. It is suggested that the drilling fluid density is increased to 1.20 sg prior to entering the Tanuma and Nahr Umr Formations. Prior to 50 m or while entering to Nahr Umr; bridging pills (48 sec/qt) inclusive of sealing and CaCO3 materials are suggested to be pumped not only while drilling but also when pulling out of hole for any operation in open hole without bit under rotary table. Equivalent Circulating Density (ECD) shall be monitored. The following ECD management practices are best practices: Slow pump start-up and breaking of gels, Max flow rates only as required for good hole cleaning – reduce flow rates when drill collars are in front of shales, controlled drilling fluid, minimized surge/swab pressures. Use of two roller reamer in BHA at optimum distances to reduce the sloughing and prevent tight spots during reaming, back-reaming and wiper trips. After having drilled each stand the fresh interval must be reamed good enough after ample circulation. In the event of stuck pipe across a limestone formation pumping 22% HCl/HF acid to cover the annulus area from the bit the top of the carbonate formation and make sure that the acid pill soaks for duration of two hours while attempting to rotate the string. CONCLUSIONS One of the important findings of this paper is the fact that the wells shall be time limited to open-hole exposure and the troublesome formations shall be cased off as early as practicably possible. The drilling crew shall be very much attentive to all of the indications the well could give, and read carefully the meanings and take the necessary actions in a timely manner to prevent occurrence of stuck pipe events. In order to avoid facing costly and long fishing/remedial operations preventing the occurrence of stuck pipes is to share the experience that sees common acceptance not only among the drilling contractors but also the operators. It is very important to know that it only requires small cuttings volume, to get stuck and packed off while pulling upwards. Each stuck pipe incident or event must be analyzed in detail, and the root cause must be put on the table by drilling engineering experts. The necessary recommendations must be issued at the earliest and the key findings must be distributed to all parties involved in the Field development. The well should be treated accordingly, and the indications being given by the well shall be well understood in a timely and practical manner. The response of the driller is going to be the key whether to face the stuck pipe event or not, unless the event was inevitable due to a well design issue or other technical reason. Stuck pipe events are evitable with continuous efforts including conducting meetings with crew prior to the commencements of the shifts, studying the offset wells in detail, understand sticking mechanisms, and taking the actual stuck-pipe events as lessons learnt. The hydraulic parameters must be more effective ensuring the hole cleaning is not an issue together with preventing the circulation loss occurrence. One of the solutions to prevent drilling through the fractured/depleted reservoirs is the application of Managed Pressure Drilling, which may be studied in a separate dedicated study to understand the feasibility. NOMENCLATURE C = coefficient of friction dc = largest cutting diameter (ft) dcuttings = cuttings diameter (in dh = wellbore diameter (in) hm = mud cake thickness (in) g = acceleration due to gravity (ft/sec2) MW, df = drilling fluid density (sg) L = Contact length of pipe with the cake (ft) Pd = differential pressure (psi) rh = radius of the borehole (in) rdc = radius of the workstring (in) Vmin = minimum annular velocity (ft/min) Vs = slip velocity (ft/sec) T = tension (klbs) W = projected width of the contact area (in) s = cuttings density (lb/ft2) m or df= mud density (lb/ft2)  = mud viscosity (cps) ABBREVIATIONS AV = Annular Velocity BHA = Bottom Hole Assembly ECD = Equivalent Circulating Density HI-VIS = High Viscosity LCM = Lost Circulation Material MW = Mud Weight NPT = Non Productive Time POOH = Pull Out of Hole PPFG = Pore Pressure Fracture Gradient SPP = Stand Pipe Pressure Q = Flow Rate WOB = Weight on Bit REFERENCES [1] G.T. 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Cocking, “A task force approach to reducing stuck pipe costs,” SPE/IADC Drilling Conference, Amsterdam, Netherlands, 1991, pp. 1-6. [18] G. Yarim, U. Rodney, M. Richard, T. Alejandro, and P. Church, “Stuck Pipe Prevention--A Proactive Solution to an Old Problem,” SPE Annual Technical Conference and Exhibition, Anaheim, CA, 2007, pp. 1-12. [19] J. M. Guzman, M. E. Khalil, N. Orban, M. A. Mohiuddin, J. Verma, and S. Ganda, “Stuck-Pipe Prevention Solutions in Deep Gas Drilling; New Approaches,” SPE Saudi Arabia Section Technical Symposium and Exhibition, Al-Khobar, Saudi Arabia, 2012, pp. 1-9. [20] M. B. Al-Saeedi,. Al-Mutairi, M. Al-Khaldy, and T. Sheeran, “Fastest Deep Well in North Kuwait: Case History of Raudhatain 206,” SPE/IADC Middle East Drilling Technology Conference and Exhibition, Abu Dhabi, UAE, 2003, pp. 20-22. APPENDIX-A: PPFG (Pore Pressure Fracture Gradient) Chart of Southern Iraq