Fracture Closure Stress (FCS) and Lost Returns Practices

Abstract

Abstract In the mid 1990's the operator's lost returns mitigation practices were modified to align with a simple, but robust, rock mechanics concept that described the manner in which LCM and other products work. The model holds that regardless of the type of treatment used, integrity is increased by widening the fracture to increase its closing stress. Field practices have been changed to enhance a variety of mechanisms that may play a role in this process. Depending on the product, these mechanisms may include development of an immobile mass, deposition of solids in layers, development of viscous resistance, and the deposition of adhesive solids. The paper presents the development of the rock mechanics perspective, the resulting Fracture Closure Stress (FCS) Practices, a treatment selection guide, worldwide results, and discussion of the key factors that continue to limit performance in some situations. Introduction Major losses most often occur through fracture propagation. Although other forms of loss, such as vugular, matrix seepage, and filtrate loss may also be of concern, fracture propagation losses account for over 90% of the operator's lost returns expenditures. The Fracture Closure Stress (FCS) Operational Practices that were developed in the mid-1990's were based on a unique perspective gained from studying the simple linear elastic model of rock behavior that was introduced in the 1950's.1 Field practices were modified in a manner that the model suggested would enhance performance, and management committed to executing the practices, even though they differed from industry norms. The treatment success rate improved significantly. In 2000 a major reorganization of the Drilling group enabled the FCS Practices to be extended uniformly to the worldwide organization, including drilling environments as diverse as the inland U.S., Gulf of Mexico, Malaysia, North Sea, West Africa, and Canada. The worldwide results for a one-year period are shown in Figure 1. The success rate in permeable formations in 2003 was 100% and increases in integrity of over 3600 psi were achieved with conventional, inexpensive products. Rig time associated with lost returns has been reduced and casing strings have been eliminated. Unfortunately, Figure 1 also shows that the success rate in low or damaged permeability continues to be limited. For reasons that are discussed below, lost circulation materials (LCM) do not work in impermeable formations, and alternative treatments have not been as predictable. This paper does not contain new rock mechanics theory, only some unique perspectives developed from relatively old concepts. Because it has not been possible to fully simulate the variety of fracture behaviors that are observed in the field, conclusions are drawn from field data and analysis of 100–130 lost returns events per year. In the past, this type of empirical experience has not necessarily lead to learning. But, because lost returns events are now viewed from within a uniform model of fracture behavior, it has been possible to learn from each event in a way that those learnings can be scaled to meet conditions elsewhere. Uniform training in the FCS Model in a required two-day workshop for engineers and rig supervisors has also contributed to the ability of the organization to learn from each event and move forward. Model of Fracture Behavior A fracture is opened when the wellbore pressure is sufficient to overcome the sum of 1) the stress holding the rock closed and 2) the tensile strength of the rock.4 The common intuition among field personnel is that integrity is related to rock strength or hardness. Rock hardness does relate to compressive strength, but lost returns occur when the wellbore wall is placed into tension, not compression. Sedimentary tensile strengths are typically less than a few hundred psi, even in formations with very high compressive strength. In contrast, the stress holding the borehole closed is typically around 75–85% of overburden (i.e., 8000 psi at 10,000 ft TVD), which is quite large relative to the tensile strength. Consequently, integrity is primarily a function of stored stress, and not the strength of the rock itself.