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Cement is used to hold casing in place and to prevent fluid migration between subsurface formations. Cementing operations can be divided into two broad categories: primary cementing and remedial cementing. The objective of primary cementing is to provide zonal isolation. Cementing is the process of mixing a slurry of cement, cement additives and water and pumping it down through casing to critical points in the annulus around the casing or in the open hole below the casing string. Zonal isolation is not directly related to production; however, this necessary task must be performed effectively to allow production or stimulation operations to be conducted.
You must log in to edit PetroWiki. Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. The last of the cement slurry, generally the highest strength cement designed to be left across the casing shoe.
You must log in to edit PetroWiki. Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. A term usually meaning flash setting of a cement or other material. May be intended or unintended.
The implementation of a well integrity management system in a mature liquid hydrocarbon storage field brought to light essential questions about the present condition and likely evolution of the barrier elements in the years to come. Are there defects in the cement sheaths that were not detected at the time of site construction more than 50 years ago? Are there aging processes, such as casing corrosion or cement degradation, that could limit the field's useful life or even present an immediate risk? And, finally, what were the exact properties of cement and casing, given that the information available in drilling reports is very limited?
At the start of the campaign, the perceived need and available technology meant using a logging tool requiring pressure, and therefore the inspection would typically involve 2 workovers and a period of 3 weeks. However, tool selection focused on the actual aging risks, together with the use of new transmitters and relentless operational optimization improved operational efficiencies allowing two wells to be logged in two consecutive days, with important savings in direct and indirect cost as well as minimization of operational risk. The cost reduction did not come at the expense of actionable information: the wireline tools selected, together with detailed preparation and carefully supervised execution led to outstanding data quality being collected.
The implementation of a consistent method of absolute log calibration and quantitative evaluation allowed us to characterize cement properties and defects, as well as their evolution with time. A salient signature, common to most storage sites in salt, is high cement quality across the salt, with a sudden unexpected, partial loss of bond across the anhydrite that overlies the rock salt formation. This was recognized as a benign form of "sulfate attack": The precipitation of secondary ettringite within the cement matrix results in expansion and thus debonding across the stiff anhydrite, whereas creeping salt pushes cement back on the casing. Lead cement densification was also observed near shallower aquifers, also rich in sulfates, with clamping provided by marls in this case. After optimizing the logging suite, it was concluded that there was no risk of erosion, wear or corrosion, external as well as internal. The analysis also improved understanding of cement behavior across salt and the role of creeping formations.
A streamlined approach at integrity assurance, whereby the right questions are asked by the management system, optimum inspection protocols are selected and carefully carried out, and acquired data is processed using advanced quantitative techniques, allowed us to understand characteristics and dynamics of barrier elements and to conclude that safe operation of the site is possible and that there is no fixed life span.
Orprasert, Choosak (Mubadala Petroleum) | Prasongtham, Pattarapong (Mubadala Petroleum) | Abu-Jafar, Feras (Mubadala Petroleum) | McManus, Ian (Mubadala Petroleum) | Thanasarnpisut, Viraphon (Schlumberger) | Johri, Ambuj (Schlumberger) | Duong, Anh (Schlumberger) | Shafie Jumaat, Mohd (Schlumberger)
Effective zonal isolation in wellbores with a challenging mud removal environment is well known to be very difficult to achieve. In wells at the technical limits of Non- Aqueous Fluid (NAF) removal prior to cement placement, cement bond quality and hydraulic isolation can be compromised by leaving channels behind the casing, which can result in several long-term well integrity issues. An Interactive Cementing System (ICS) is developed through special experimental methodologies to mitigate mud channeling issues and improve zonal isolation, by immediately interacting with any residual mud channels left in the well after cement is in place, hence reducing the permeability of mud channels and sealing off microannulus gaps.
Casing centralization is considered to have the greatest influence on mud removal efficiency because it directly affects the flow movement on each side of the wellbore. Mud removal has been studied from numerical simulations, laboratory experiments, and field results, and these show that good mud removal can be achieved only when adequate casing standoff is achieved during cementation. In modern wells where there are many operational restrictions and limitations, especially in highly deviated and horizontal wellbores, final cement designs may not allow good casing standoff and thus not all of the best practices for effective mud removal can be applied.
The objective of the innovative cement system is to have a design that interacts with residual mud in the annulus to "fix" the channels, thereby enhancing cement bond quality and zonal isolation. Two detailed case histories of the application of this technology in the development campaign showed visible improvement in cement bond logs using the ultrasonic imaging tool as compared to offset well that was cemented using a conventional cement system. After two successful implementations, the ICS was selected as the cement system of choice for wells with challenging mud removal.
Loss circulation is encountered frequently while drilling fractured carbonate reservoirs in specific field. The field practice was attempting to cure losses and if incurable, drill blind to total depth (TD) followed by run and cementing of the liner. The interval from loss zone to liner top was covered by the cement squeezed from liner top. The require time to try to cure the lost circulation zone plus squeezing cement job was approximately 15 days. Several optimization initiatives were implemented to reduce this time to less than seven days.
There were at least eight round trips carried out in different ways by different operators to complete the operation of attempting to cure the losses and a liner top squeeze. The engineering team evaluated this for potential optimization, first to identify whether or not losses need to be attempted to be cured to save the time lost on unsuccessful attempts. Second, to analyze the lessons learnt and build on that optimization strategy to reduce the number of trips Lastly to rework the cement slurry design to reduce the number of attempts to squeeze liner top.
As such a detailed strategy was formulated regarding when and how to cure losses followed by an optimized procedure for liner top squeeze which saves three round trips. Further, the liner top squeeze operations previously took multiple attempts of squeeze before a successful pressure test was achieved. Based on the lessons learnt, the slurry design was optimized from several aspects including, slurry density, rheology, thickening time and the pumping and displacement procedure was created which helped to reduce the number attempts from six to only one. Another optimization implemented was enabling the loggers perform pressure pass for cement evaluation by the utilization of tractor instead of conventional (Tough Logging Conditions) TLC which not only saved time but also depicted better the condition of cement behind liner. Finally, a robust risk assessment encompassing all possible contingencies for expected issues was incorporated.
The optimized liner top squeeze strategy has been implemented at five wells with 100% success, reducing the overall operation time from more than two weeks to less than one week while improving cement quality behind liner to ensure zonal isolation as per requirements.
This paper provides details of how the cement slurry, operations sequence and tools selection were enhanced well by well based on continuous improvement. Since cementing liners across loss circulation intervals exists in most of the carbonate reservoirs worldwide, this paper will help to achieve better zonal isolation in losses environment with lower cost and lesser time.
In our plant, sulphur is recovered from gas in Sulphur Recovery Units and stored in Liquid Sulphur tanks, before granulation and pelletizing for shipment. Underground concrete storage pits form an important element of the Sulphur Recovery, Treatment / export cycle, providing an intermediate containment to ensure that molten sulphur temperature is maintained within the operating range. These concrete pits tend to deteriorate rapidly due to high temperatures and contact with concentrated sulphur. This paper discusses the pit design and construction practices, and efficient repair/maintenance techniques of ageing containments to ensure improved service life while optimizing maintenance costs.
The structure consists of reinforced concrete walls, base and top slab, which supports the Sulphur transfer pumps and associated piping / instrumentation. At bottom, steam coil system is provided where Low Pressure steam is used as heating medium to maintain molten Sulphur at high temperatures.
Primarily two concepts are followed for protecting Sulphur pits: (a) Internal surface lining with acid resistant / refractory bricks and insulation, and external waterproofing, or (b) Externally applied waterproofing and thermal insulation, with no internal coating or lining.
Since exposure to high Sulphur concentration and high temperature renders the concrete susceptible to sulphate attack and deterioration, it is essential to carry out periodic inspection and maintenance works in these pits. The top slab is susceptible to higher corrosion due to contact with vapours.
Evaluation of existing pits with above designs revealed contrasting conditions. Refractory / brick lined pits indicated severe deterioration of lining, with traces of corrosion in concrete surfaces. Underside of slabs also showed deterioration of tiles, however, slab concrete was relatively in better condition. Unlined pits revealed the concrete surfaces of walls and base slab to be in reasonably good condition. However, underside of top slab was found to have severe deterioration, beyond repair mandating replacement. Physical and chemical tests indicated reasonably good condition of reinforcement steel, retention of concrete strength, and no loss of concrete cover. However, sulphate content near the surface was found to exceed code permissible limits.
Repair procedures for lined and unlined pits vary, with primary focus in lined pits being selection of appropriate lining for Sulphur resistance and thermal insulation, while for unlined pits, the main concern is selection of suitable concrete mix design and ensuring proper compaction and curing.
Deterioration of concrete in process critical structures like liquid sulphur pits is an important issue, which could potentially impact the Sulphur recovery and export process. Both lined and unlined pits are found to be suitable for the purpose, however, each has its own pros and cons. Maintenance and repair procedures need to be in place specific to the type of design and construction of these pits, to improve service life.
Cementing of production casing in the Northern Iraq poses challenges to the cement sheath integrity due to mechanical and thermal stresses induced in the well life. The problem is further aggravated due to narrow window between pore pressure and fracture gradient. The acid-prohibitive cement system with improved mechanical properties was developed to mitigate the effect of induced stresses. The job was executed with operational optimization and zonal isolation was achieved.
Based on the operator's well testing and multi-stage high-rate well stimulation plan, the stress modeling was carried out to determine the optimum mechanical properties. The 19.6 ppg heavyweight cement system with a flexible thermoplastic polymer was designed to achieve the required Young's modulus and Poisson's ratio. Since the density and friction pressure hierarchy could not be met due to the narrow window between pore pressure and fracture gradient, therefore, the slurry rheological properties were optimized for effective mud removal. The pumping parameters were adjusted to maintain the primary well control during the cementing operation without compromising displacement efficiency.
The approach was implemented without any operational issues in the 9-7/8" production casing and 7" liner cementing. Following the job completion and waiting-on-cement time, the 9-7/8" casing was successfully pressure tested with a surface applied pressure of 2,000 psi and a well fluid of 1.78 SG. The isolation scanner cement evaluation confirmed the zonal isolation along the open hole of both the 9-7/8" intermediate casing and the 4½" production liner. Finally, the multi-stage high-pressure stimulation operations were performed during the completion/testing stage with no sign of communication between the different zones. The application of heavyweight acid prohibitive flexible slurry helped the operator to isolate the different zones of interest that were less than 10 m apart and retained the integrity of the seal throughout the high-pressure stimulation operation. Well is open to production without any annular pressure, thus, saving the operator's time and cost on the remedial cementing operations.
The proposed solution will help operators to ensure long-term zonal isolation in the HTHP wells which are subjected to dynamic pressure and temperature changes in the post slurry placement phase. The operators can also avoid the time and money on expensive remedial operations.
To determine which salt-based cement system [potassium chloride (KCl) or sodium chloride (NaCl)] was suitable for cementing across halite and anhydrite salt sections in West Africa, eight slurry recipes were tested to assess how formation salt contamination would affect slurry properties. The formation salt used for testing was sampled from a deepwater, presalt well in Angola. The recommendations developed from the laboratory study were implemented in 10 projects across West Africa over 5 years with 100% operational and well integrity success.
A candidate deepwater well was selected in which the surface and intermediate strings penetrated salt formations. A total of four slurry designs (a lead and tail slurry used on each casing string) was programmed. Each slurry was designed and tested as two distinct systems using KCl and NaCl salt respectively, yielding a total of eight slurry designs. Using the methodology and data presented by Martins et al. at the 2002 IADC/SPE Drilling Conference (SPE-74500-MS), the mass of dissolved formation salt that each slurry may receive during placement was estimated and duly incorporated into each slurry design. Subsequently, the salt-contaminated slurries were tested and compared with the properties of the initial uncontaminated slurries. Based on these results, conclusions were then made on which salt slurry system (KCl or NaCl) exhibited better liquid and set properties after contamination with formation salt. Subsequently, this knowledge was applied to 10 projects across three countries in West Africa.
This study showed that when the contact time of liquid cement slurry to salt formation was low—typically when the salt formation interval across which the cement slurry flowed was less than 100 m thick—the level of formation salt dissolution entering the slurry during placement was limited. In this case, a KCl salt-based slurry delivered improved liquid and set properties as compared with a NaCl salt-based slurry. In the field, this knowledge was applied in all oilfield projects cemented by an oilfield service company between 2015 and 2020. This included deepwater, shallow offshore, and onshore wells. All related salt-zone cement jobs, including sidetrack plugs, placed across the salt formations were successful on the first attempt.
In an absence of industry consensus around salt-formation cement slurry design, this paper validates a guideline for West Africa, based on results from laboratory testing and 5 years of field application. In contrast to current literature that recommends only NaCl salt-based slurry designs across halite or anhydrite salt intervals, this work demonstrates that KCl salt-based slurry systems can effectively be used to achieve well integrity where a halite or anhydrite salt interval is less than 100 m [328.1 ft] thick.