Abstract

Transient heat transfer is considered frequently in operations involving thermal flooding for EOR/IOR, pressure pumping during reservoir stimulation, geothermal wells, and HP/HT environments in general. These operations have a critical impact on the integrity of wellbore cementing. Generally, engineers use complex simulation software packages with finite element analysis (FEA) to tackle these kinds of problems. This methodology is time-consuming and costly.

In this paper, we propose a new solution with an approximation method to analyze transient stress responses to thermal variation in well cementing. We decouple the differential equations from heat transfer, stress-strain relations and continuum equation. The transient temperature distribution is solved from an approximation method, and then the stress distribution profiles are obtained by superimposing transient temperature effects on thermal stress and strain profiles.

The results from our simulation are presented in our case studies. The transient temperature distribution profiles in casing-cement-formation are provided by this simulator. The transient radial stress and tangential stress distribution in a cement sheath system as a function of radial distance are discussed. This approach allows field engineers to conduct quick analyses without loss of simulation quality within a certain acceptable degree.

Introduction

Cementing in the annulus between the casing and the formation plays an important role during drilling and completion phases in our exploration and production industry. Most recently, well cementing has been applied in conjuction with carbon capture storage (CCS), which tries to address high concentration CO2 emission in atmosphere.

As energy demands increase globally, more advanced technology is applied in unconventional oil and gas reservoirs under harsh downhole conditions. Temperature variations are usually large during drilling, completion and well stimulation for the exploration and operation in unconventional reservoirs. After the cement sets, the cyclic thermal loading may result in cement zone isolation failure if the cement material is degraded and the mechanical properties are changed. Even if the cement is placed initially with proper mechanical strength and a strong hydraulic seal at the casing/cement and cement/formation interfaces, it may be subjected to failure because of transient heat transfer inside the casing, cement and formation. In addition, pressure variations and mechanical shock, such as perforating can weaken the bond strengths at the casing/cement or cement/formation interfaces. Cement shrinkage is also a key factor for creating some circumferential fractures at the cement/formation interface[1]. In recent years, more studies have focused on the stability and capability of cement sheath to ensure well integrity throughout the lifetime of well. In general, cement mechanical failure (compressive strength failure and tensile strength failure) and debonding of cement from casing and cement from formation are the main reasons for the loss of zonal isolation.

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