Step-Change in Cement Design Assures Well Integrity Under Dynamic Stress

Abstract

Abstract The Kohat Plateau in Pakistan features major Gas and Condensate producing fields with majority of the blocks owned and operated by two of the country's biggest E&P Operators. Operators are known to frequently encounter severe drilling challenges, and over time, have devised certain technology solutions that have helped curtail the major drilling challenges to a good extent, helping improve the time and cost expended in reaching the target depths. However, the problems arising after the Drilling cycle – foremost being Sustained Casing Pressure (SCP) – require expensive remedial treatments and loss of production; this paper presents a solution to this particular challenge of SCP including in-depth cause analysis, treatment design and implementation. In the analysis stage, the drilling data of wells experiencing SCP in one of the Casing annuli was reviewed to pinpoint possible areas of compromised well integrity. It was observed that sections having good Cement bonds and sufficient Top of Cement in the annulus also became compromised over time, indicating operations conducted after the drilling phase could be responsible for the annular fluid migration, and hence SCP. It must be noted that extensive drill-stem tests to assess the reservoir economics as well as production pressures generate significant dynamic stress on the well structure. Through the use of custom stress-analysis software for annular Cement, it was concluded that it is this stress that causes the primary isolation of the constructed well, the annular Cement, to fail allowing annular migration of formation fluids. A new class of stress-resistant, auto-repair Cements was seen to resist any mechanical failure under stress enabling Operators to drill wells with lasting structural integrity. Mechanical properties including Young's Modulus, Poisson Ratio and Annular Expansion co-efficient were analyzed to design a Cement system capable of withstanding the applied stress. The first in-country application was conducted as the Intermediate-section Cementing in a new well. The system exhibited good isolation with average 10mV amplitude (free pipe amplitude 51 mV) in Cased Hole bond logs and no annular pressure is observed till date in the section more than 02 years after the primary cementing operation. This design approach was subsequently extended to other fields, with successful applications that reduced, and in certain cases, eliminated the need for remedial work. By presenting a detailed field application of flexible and self-healing Cement, the paper puts forth a general, stepwise approach for the selection of a Cement design that promises lasting annular isolation and minimizes the risk of communication behind the casing.