Preventing Lost Circulation Requires Planning Ahead

Abstract

Abstract "Pay now, or pay later" fits in many industries and circumstances. In well construction, ideally the operator wishes to be in a position to decide which of these alternatives is the most economic. Engineered solutions, designed to improve wellbore strength and reduce drilling non-productive time due to lost circulation, are directed at managing wellbore stresses (WSM). This fully engineered approach should include means to help prevent lost circulation as well as stop losses. Prevention of lost circulation by improving the wellbore strength is accomplished by designing and applying borehole stress treatments that increase the hoop stress around the wellbore. The technology development within the industry that led to the applications of these concepts will be discussed. Proprietary hydraulic design software (HDS) can predict the equivalent circulating density (ECD) over an interval in one module, calculate the width of a fracture that may be initiated, and select and design a proper material and particle size distribution that can efficiently prop and plug that fracture in a second module. A third module then predicts the change in rheology resulting from the addition of the specialized lost circulation materials, which then is cycled full circle back to update the ECD calculations. Contingency chemical sealant (CS) treatment applications are systems designed to react with the drilling fluid itself to create highly viscous and cohesive sealants in the wellbore that are displaced into the lost circulation fractures. Drilling-fluid-reactive systems are not dependent on temperature or pressure, thus removing a significant amount of placement uncertainty present with cross-linked systems. This combination of planning and application tools allows the operator to make decisions ahead of time during the "drilling the well on paper" phase as to which approach is the most economic - prevention (pay now) or remediation (pay later). Overview Lost circulation is one of the biggest contributors to drilling non-productive time (NPT), and it is the most difficult segment of drilling in which to make economic decisions. Estimations of economic impact in this segment vary widely, but it is safe to say that it represents a very large portion of the total non-productive expense for drilling a well. As rig rates increase, the economic impact of NPT increases as well. Therefore, any technology that reduces drilling NPT can translate into millions of dollars in reduced drilling costs. To address this problem, engineered solutions designed to improve wellbore strength and reduce drilling NPT caused by lost circulation were developed. This WSM service provides a fully engineered approach to lost circulation problems that incorporates both unique planning software and materials. Lost circulation planning includes both prevention and remediation methods. While it is critical that losses be stopped once they occur, it is equally important that they be prevented because problems prevented represent money never expended. One important part of the preventive plan is the design of "borehole stress treatments". The goal of these treatments is to increase the "hoop stress" in the near-wellbore region to improve the wellbore pressure containment ability. Wellbore Stress Theory Conventional fracturing theory predicts that lost circulation may occur when the tangential stress at the borehole surface exceeds the tensile strength of a rock. However, this conventional theory could not explain why lost circulation occurs more frequently when oil-based drilling fluids are used. Based on results from joint industry project DEA 13 conducted in the mid-1980s to answer this question, it was proposed that a stable fracture containing drilling mud with solid and gel components can exist and that lost circulation occurs when the fracture becomes unstable.1 This ultimately led to the conclusion that lost circulation mitigation could be enhanced by carrying materials in the drilling fluid that were of a proper size, concentration and type.2 The most significant result was not that lost circulation could be controlled by these treatments, but that the resistance to lost circulation (increased wellbore strength) could be enhanced significantly.3