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Kopp, Kevin (Chevron) | Reed, Shawn (Halliburton Energy Services, Inc. ) | Foreman, Jay (Halliburton Energy Services, Inc. ) | Carty, Brian (Halliburton Energy Services, Inc. ) | Griffith, James (Halliburton Energy Services, Inc. )
Abstract Data from six deep gas wells in Wyoming indicate that foamed cement outperforms conventional cement for zonal isolation. Two of the six wells were cemented across the production zones with conventional non-nitrified cements. These wells experienced outer-zone communication after the stimulation treatments. In the other four wells, the foamed-cement sheath provided zonal isolation even after the sheath was perforated and stimulation treatments were performed. The case histories described in this paper provide an opportunity to compare the performance of foamed cement to conventional cement on similar wells within a particular geographic area. Large-scale laboratory testing has shown that foamed-cement is ductile and can deform as casing is pressurized, but will not crack like non-nitrified cement. These test results were confirmed by the results obtained in the six case-history wells. In the first two wells that were cemented, the operator used the conventional high-strength, non-nitrified cement across the formations. Tracer tags on the stimulation treatment showed that zonal isolation was not achieved and that the stimulation treatment communicated between the high- and low-pressure zones. The operator elected to use foamed cement for the last four wells to help obtain zonal isolation. In these wells, the stimulation treatment remained in the zone, little fracture growth occurred outside the target formation, and communication did not occur between the high- and low-pressure zones. This paper provides details about the slurry design and job execution on the six case-history wells, and also presents the postjob analysis that verifies the conclusions. Foamed Cement Properties and Applications Introduction Foamed cement's tensile strength, ductility, and displacement properties have made it especially useful in several zonal-isolation scenarios. Although the initial cost of conventional cement can be less than that of foamed cement, the improved zonal-isolation capabilities of foamed cement often provide substantial cost savings over the life of the well. The following section discusses both the advantages of using foamed cement for zonal isolation and its economic value compared to flexible cement. Mud Displacement/Fluid Influx and Migration Control During pumping operations, foamed cement can develop higher dynamic-flow shear stress than conventional cements, increasing its mud-displacement capabilities. A system that consists of cement slurry injected with nitrogen gas can be optimized for individual well conditions. Slurry density, which is determined by gas content or quality (the porosity of the set cement), depends on the pump rate of the base slurry, foamer and stabilizer injection rates, and nitrogen rate. Computer programs help optimize slurry design and predict job-placement pressures. The gas used to foam the system continues to expand while the cement volume decreases, allowing slurry pressure to remain almost constant during the system's transition period. Consequently, the system effectively controls gas migration and formation-fluid influx, which limits migration channels in the set cement sheath. Ductility The brittleness or lack of ductility of conventional cements has been identified as one of the primary failure mechanisms of primary cement jobs. Flexible cement systems have been used to successfully combat and prevent cement-sheath cracking. However, flexible cement systems, which incorporate vulcanized rubber, are very costly and require extreme conditions for justification of their technology. Introduction Foamed cement's tensile strength, ductility, and displacement properties have made it especially useful in several zonal-isolation scenarios. Although the initial cost of conventional cement can be less than that of foamed cement, the improved zonal-isolation capabilities of foamed cement often provide substantial cost savings over the life of the well. The following section discusses both the advantages of using foamed cement for zonal isolation and its economic value compared to flexible cement. Mud Displacement/Fluid Influx and Migration Control During pumping operations, foamed cement can develop higher dynamic-flow shear stress than conventional cements, increasing its mud-displacement capabilities. A system that consists of cement slurry injected with nitrogen gas can be optimized for individual well conditions. Slurry density, which is determined by gas content or quality (the porosity of the set cement), depends on the pump rate of the base slurry, foamer and stabilizer injection rates, and nitrogen rate. Computer programs help optimize slurry design and predict job-placement pressures. The gas used to foam the system continues to expand while the cement volume decreases, allowing slurry pressure to remain almost constant during the system's transition period. Consequently, the system effectively controls gas migration and formation-fluid influx, which limits migration channels in the set cement sheath. Ductility The brittleness or lack of ductility of conventional cements has been identified as one of the primary failure mechanisms of primary cement jobs. Flexible cement systems have been used to successfully combat and prevent cement-sheath cracking. However, flexible cement systems, which incorporate vulcanized rubber, are very costly and require extreme conditions for justification of their technology.
Results from a detailed program undertaken to characterize the properties of both a curing and a cured cement sheath are presented. These properties include (a) hydration volume change during curing, (b) mechanical properties and (c) endurance limit, and are used to design cement systems to withstand stresses resulting from well operations.
Industry evidence indicates a significant fraction of wells are exhibiting annulus pressure during the life cycle of the field. Ideally, the drilling fluid in which the casing is run should be displaced, and the cement slurry placed in the annulus covering the critical zones for zonal isolation. The cement sheath placed in the annulus should have optimum properties to withstand stresses during well operations. However, inadequate post-curing sheath properties can negate a successful placement operation.
The hydration volume change during cement curing is measured for different cement systems. Both the so-called internal and external volume changes are measured. Procedures are discussed to measure these properties under downhole conditions. The implications of these values on downhole performance of cement sheath are also discussed.
Small-scale tests were conducted to determine the effect of changes in pressure and temperature on cement sheath characterization. The results from these tests, along with the relation to the properties measured are presented and discussed.
Endurance limit is defined, as with a metal, as that percentage of the yield to which the cement sheath can be subjected to and still withstand a large number of load cycles. Test procedures and results for different cement systems will be presented and discussed along with the implications on downhole performance.
The work presented in this paper can be used to characterize cement sheath properties under downhole conditions and design cement sheath to withstand stresses during the life cycle of the well.
The main purpose of a primary cementing job is to provide effective zonal isolation for the life of the well so that oil and gas can be produced safely and economically. To achieve this objective, the drilling fluid should be removed from both the wide and narrow annulus, and the entire annulus should be filled with competent cement. The cement should meet both the short-term and long-term requirements imposed by the operational regime of the well. Typical short- and long-term properties required from the cement are listed in Table 1.
(Table in full paper)
Traditionally, the industry has concentrated on the short-term properties that are applicable during the cement slurry stage. This is necessary and important for effective cement slurry mixing and placement. However, the long-term integrity of cement sheath depends on its mechanical properties such as Young's modulus, tensile strength, resistance to downhole chemical attack, etc.
After placing the cement slurry in the annulus, if there is no immediate migration of fluid to the surface, it is likely that short-term properties such as density, rate-of-strength development, and fluid loss of the cement slurry have been designed satisfactorily.
However, recent experience has shown that after operations such as completing, pressure testing, stimulating, and producing, the cement sheath could lose its ability to provide zonal isolation.
Cementing is a relatively low component of overall well construction cost.Unsuccessful first time cementing operations
impact drilling Non Productive Time (NPT) but much more critically, and oftenover-looked, is its long term effect on well
operability and well integrity. The cost of loss of hydrocarbon production andearly onset of water production due to poor
cement isolation is thought to be huge, but is rarely measured by operators dueto the technical disconnect that can occur
between an operator's drilling and production staff. Similarly there has beenlittle quantification of the cost associated
with sustained casing pressure on well operability and long termabandonment.
Problems during well construction which result in direct NPT have been wellresearched. There is agreement about the best
practices for topics such as setting cement plugs and preventing gas migrationHowever, the industry is now waking to the
legacy of a large number of wells with sustained casing pressure as a directconsequence of poor cementing. In many cases
this is related to poor cement placement, but it can also be attributed to longterm cement durability. Many wells are now
being designed for >10 year life of well and long term cement isolation isbecoming a major consideration.
This presentation details some of the generally agreed best practices forprevention of drilling non productive time related
with setting cement plugs and fluid migration after cementing.
It will also address:
* Issues impacting long term zonal isolation.
* Minimum engineering design requirements for successful primary cementing andlong term isolation.
* Balancing good drilling fluid performance and successful primarycementing.
Abstract Best Practices for cementing are well established in the industry however there remains the continuous challenge of effectively employing these best practices in balancing the need for drilling performance and the need for delivering the best possible well to production. For multi-zone monobore completions where the cemented tubing annulus is the single means for zonal isolation, this becomes a critical goal. Such is the case with the majority of wells drilled in the Gulf of Thailand. Desire to meet this challenge led to the creation of a full-circle process that both provides the methodology and initiates the inter-disciplinary communication necessary to improve zonal isolation and the added value it will bring. Components of the full-circle process employed in Thailand include: creating a cementing "scorecard" to focus operations on cementing best practices; forming a cross-disciplinary team of drilling, completion; production and contractor personnel to review bond logs, adjusting drilling practices; and communicating to all involved in the process. An integrated team approach developed within the context of the full-circle process has successfully linked the goals of the Subsurface Operations and Production Departments resulting in the creation of more production opportunity and maximized recovery rates. The CBL Review Team (CBLRT) created in late 2002 consists of drilling, completions and reservoir engineers, along with logging and cementing experts from the representative service providers. In many cases members of the Asset Management team also attend the regular weekly meetings. To date, the team has reviewed over 450 wells. This paper presents the full-circle process model and shows how improving zone isolation, evaluation methods and field results can be adapted for application in other operating environments. Background Unocal comenced drilling in Thailand during the 1960's with first production coming in 1981. As of April 2004, the company has produced 5,527 bcf of gas and 198 mm barrels of condensate from several large offshore gas fields. First oil production occurred in July 2001 and as of April 2004, 14.1 mm barrels of oil has been produced. The company now has over 2,000 wells on 95 wellhead platforms. Current daily production averages are ~1.2 bcf gas, ~42,000 bpd condensate and ~18,000 bpd oil. In general, a batchdrilled, slimhole, mono-bore completion is adopted for the GOT. The surface 9–5/8" casing is set in 12–1/4" hole at approximately 1,000 ft. TVD, followed by the 7" casing set at the pressure ramp near 5,000 ft TVD in 8–1/2" hole. During this section most of the directional work is accomplished. The final phase is a 6–1/8" deviated tangent section that intersects as many as 30 pay sands. Porosity, permeability and water saturation in theses sands vary tremendously as well as the reservoir pressures, which range from 1,600 to over 5,000 psi. A 2–7/8" completion string with TRSV is run into the 6–1/8" hole and is cemented in place. Simplifying operations via batch drilling and the use of the slimhole, mono-bore concept has resulted in substantial cost savings. This has been the driver for more development wells and higher production rates, which in turn has allowed further drilling into ever more challenging environments. Several drilling speed records have been set by Unocal in the GOT. In 2002, 253 wells were drilled at an average of 5.3 days per well. For 2005 Unocal's goal is to drive the drilling time down to 4 days per well in the GOT. To achieve this, several technical challenges will need to be overcome.
Mahamat Habib, Abdelkerim Doutoum (ZADCO) | Al Katheeri, Yousif Saleh (ZADCO) | Seales, Sheldon (ZADCO) | Ramdeen, Rayaz Evans (ZADCO) | Bermudez, Romulo Francisco (ZADCO) | Navas, Luis Eduardo (ZADCO) | Kapoor, Saurabh (Schlumberger) | Pallapothu, Surya (Schlumberger) | El Hassan, Azza (Schlumberger) | Jain, Bipin (Schlumberger)
Abstract Achieving well integrity relies on achieving zonal isolation among narrowly separated sublayers of the reservoir throughout a long openhole section. This requires flawless primary cementation with a perfect match of optimized fluid design and placement. In a UAE field, there are several challenges experienced while cementing production sections, predominantly due to long open holes with high deviation, use of nonaqueous fluids (NAF) for shale stability, and loss circulation issues while drilling and cementing. The need to pressure-test casing at high pressures after the cement is set and the change in downhole pressures and temperatures during well completion / production phases result in additional stresses that can further endanger the integrity of the cement. Breaking of the cement sheath would lead to sustained annular pressure and compromise the needed zonal isolation. Hence, the mechanical properties for cement systems must be thoroughly tested and tailored to withstand the downhole stresses. A systematic approach was applied that used standard cementing best practices as a starting point and then identified the key factors in overcoming operation-specific challenges. In addition to the use of engineered trimodal slurry systems, NAF-compatible spacers, and loss-curing fibers, an advanced cement placement software was used to model prejob circulation rates, bottomhole circulating temperatures, centralizer placement, and mud removal. To enhance conventional chemistry-based mud cleaning and to significantly improve cleaning efficiency, an engineered fiber-based scrubbing additive was used in spacers with microemulsion based surfactant. Furthermore, a real-time monitoring software was used to compute and monitor equivalent circulating density (ECD) during the cementing operation and to evaluate cement placement in real time. Results of cement jobs were analyzed to define the minimum standards/criteria and then to verify the efficiency of the applied solutions. The 9 5/8-in. casing / liners were successfully cemented using this methodological approach, and lessons learned were progressively used to improve on subsequent jobs. Advanced ultrasonic cement bond logging tools along with advanced processing and interpretation techniques facilitated making reliable, conclusive, and representative zonal isolation evaluation. The cement bond logs showed significant improvement and increased the confidence level towards well integrity. After establishing field-specific guidelines over 2.5 years, continuous success was replicated in every well for all the rigs operating in this UAE field.