Abstract
The rapid increase in oil and gas shale exploration has shifted industry dynamics by generating substantial unconventional resources. Advances in horizontal drilling and hydraulic fracturing have played important roles in enabling unconventional developments. However, high-pressure cycles from fracturing and stimulation operations can result in cement failure, which compromises well integrity because cement acts as the annular barricade, protecting and supporting the casing while preventing unwanted fluid communication. To address the relationship between cement performance during fracturing and wellbore integrity, this paper proposes a comprehensive methodology for designing and evaluating cement systems for improved integrity during stimulation and production cycles. A life-of-the-well design approach, comprised of computational modeling of both cement placement and its capacity to withstand long-term temperature/pressure cycles, is coupled with frequent cement-evaluation techniques (e.g., sonic/ultrasonic tools) to help ensure an optimum seal behind casing throughout the well's life.
The methodology was tested and/validated in field applications. The proposed methodology includes identifying the purpose of the cement operation, the cement system to accomplish that purpose, the type of evaluation technique(s) to be used, and the recommended evaluation frequency. The field test was performed in a horizontal well, where 23 fracturing stages were planned with a maximum pressure of 7,000 psi; production was intended to last 10 years, with maximum pressure differentials of 500 psi. These operational loads and others experienced during the wellbore construction were also considered to determine which cement system had sufficient capacity to withstand the predefined loads and provide long-term wellbore integrity. Results indicated that elastic cements are more suitable for cyclic loads attained during fracturing and production operations because of their higher strength-to-elastic modulus ratio. Finally, ultrasonic tools were used to diagnose the cement integrity after cementing was completed and before fracturing and production operations, and annually after production began, particularly if additional fracturing stages were considered. Initial integrity diagnostics indicated that ultrasonic logs showed optimum performance of the cement sheath before and after stimulation/production operations.
This methodology helps reduce the operational risks associated with poor sealant design and inadequate cement evaluation, which can result in performing unnecessary repair work, production of unwanted fluids, and premature well abandonment. Moreover, as a result of the cement's enhanced integrity and capability to withstand fracturing loads, it was determined that not using a fracturing string could result in significant economic gains to the operator.
