Abstract

Maintaining zonal isolation for the lifetime of oil and gas wells is critical. Leakage behind casing can reduce the cost-effectiveness of the well and cause health and safety risks from pressure build-up and contaminated aquifers. During the completion and production phases of the well, temperature and pressure variations can cause stresses at the cement-to-formation interface. The ability of the casing-cement system to maintain a seal at the cement-to-formation interface depends on the condition of the formation surface prior to slurry placement. The condition of a shale will depend upon the nature of the drilling fluid used, whereas the condition of a permeable rock will depend upon the presence and nature of the filtercake deposited during drilling and circulation.

In this paper we present an improved understanding of chemical interactions at the cement-to-formation interface and the factors that determine bond strength and the position of the plane of failure. For permeable formations, the role of the mud filtercake for different mud types is explored. For nonpermeable formations, the presence and effect of a residual mud film are also examined. The extent and depth to which chemical alteration of the mudcake occurs when in contact with cement are determined, together with measurements of the yield stress and water content profile of the altered mudcake. The effect of exposing swelling and nonswelling shales to inhibitive drilling fluids on bond strength is presented. Laboratory-scale test equipment and a small-scale wellbore simulator, developed for tests under realistic field conditions, are described. The flexibility of cement plays a role in bonding and is demonstrated by the simulator tests. This improved understanding allows us to confirm the key issues at the cement-to-formation interface and propose some solutions for effective zonal isolation.

Introduction

Failure of a cement sheath to provide zonal isolation can occur as a result of properties of

  • the cement-casing interface.

  • the bulk cement.

  • the cement-formation interface.

In the petroleum sector many studies have been carried out on the cement-casing interface and the bulk cement to improve the isolation efficiency of the cement in the wellbore. Less attention has been given to the cement-to-formation interface.

During the well construction process the formation is exposed to a drilling fluid. Usually, spacers and chemical washes are pumped ahead of the cement slurry for fluid separation and hole cleaning.1,2 Ideally, the well construction process leaves a formation surface to which the cement can bond and provide zonal isolation. With respect to the formation, two processes occur prior to cementing. The drilling fluid can interact with the shale surface to alter its water content and filtercake formation will occur on the permeable sections.

The authors found no detailed studies in the literature that discuss the quality of primary cementing jobs after using specific drilling fluids. However, some papers do allude to increased success of primary cementing jobs after using silicate drilling fluids.3 The advantage of silicate mud over polymer mud for primary cementing has been described in a study of the Lennox field. The silicate system appeared to improve hole gauge sufficiently to allow good primary cementing on the 9 5/8-in. section without the problem of gas migration. However, no detailed scientific study was carried out to check for the incorporation of the silicate from the drilling fluid into the cement matrix. A patent by Mondshine4 detail a proposed flow chart to pretreat the wellbore with a silicate fluid and then place cement slurry containing a divalent cation behind casing. However, there is no conclusive evidence of improved bonding. There is also some evidence that cement slurries containing potassium salts or amines are useful in front of shale sections.5 One paper which alludes to improved bonding between specialised cement formulations and sections of the Sabine Uplift uses realistic preparation and detailed microscopy.6

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