Research On The Cement Sheath Seal And Integrity

With oil and gas exploration and development work deepening and expanding, complex wells and special process wells are more and more that posing a great challenge to the quality of cementing, and the problem of cement sheath seal integrity caused widespread attention of the industry.Influence in the whole process of cementing cement ring seal integrity mainly has three aspects, In the process of cementing slurry displacement effect is not good will be formed in the wall or casing wall mud cake and later in the water in the process of mud displacement is difficult to wash clean, it provides effective channels for oil and gas channeling occurred; the cement slurry water loss, poor stability form short-term gas channeling or uneven, mixed slurry density can shortage in cement hardening process, bad will affect the strength of cement stone development cement ring seal integrity; Late process of hardened cement, cement ring need to meet the long-term durability of the specific environmental conditions, including the integrity of the cement ring are not destroyed, compressive strength not recession, not excessive expansion of volume shrinkage and the late permanent deformation. The above three,This article analyzes the efficiency of cementing replace and cement seal integrity.The problem of cementing displacement efficiency has been concerned by cementing workers for a long time, Due to the harsh conditions, complex replacing mechanism, and many affecting factors, the accurate prediction of the displacement efficiency have not been solved. This paper, based on the theory of fluid mechanics, and combining the constitutive equations of Bingham fluid, power law fluid and Heba fluid, established the displacement efficiency forecast models of power-law slurry displacing Bingham mud and Heba slurry displacing Bingham mud. These models considered deviation angle, centering degree, the density differences between mud and cement slurry, injection-substitute flow rate and other affecting parameters, and calculated the thickness of retention layers on the casing and borehole walls. Formed a Systematic prediction methods of displacement efficiency. Verifying by examples, we found using these models to predict the displacement efficiency has important practical significance for guiding the actual cementing and improving cementing quality.Cement seal integrity is a rising topic in recent years, that mainly analyzed the influence of wellbore internal pressure on the strength of cement sheath after hardening, and whether in this case would generate a micro-annulus gap, caused annulus pressure, well head gas channeling, and so on. In this paper, two cases were considered:First, we consider that the casing, formation, and cement sheath are linear elastic bodies; Second, consider that the casing, formation and cement sheath are elastic-plastic bodies. In the first assumption, we analysis the mechanical property of cement sheath under uniform and non-uniform ground pressure two different situations, established force model of three materials and gave a theoretical analytic solution. In the second assumption, we established elastic-plastic stress analysis models of these three materials, and gave the elastic-plastic solution under conditions of uniform and non-uniform ground pressure, Calculated the micro gap size of casing and cement sheath in the first cement surface, and gave judgments that whether there would be micro sheath gap and cement damagement under the working conditions of casing pressure testing and fracturing.At the same time, I use finite element software to analyze elastic-plastic stressconditions of these three materials under non-uniform ground stress.Finally, this paper introduces the prediction module of the cementing sheath seal integrity. The interface of this software is simple, operation is convenient. And with the example of a well test, we proved this software could provide some reference to cementing process designers.