The Mechanical Earth Model Concept and Its Application to High-Risk Well Construction Projects

Abstract

Abstract Many of today's well construction projects are technically and economically challenging. Examples include deepwater exploration wells in the Gulf of Mexico, offshore field development projects such as Hibernia, Newfoundland, Canada and onshore field development projects in tectonically active regions such as the Cusiana field in Colombia. Minimizing non-productive time associated with wellbore instability and unexpected pore pressure regimes reduces the risk of dangerous accidents and is required to complete the well on time and within budget. Minimizing non-productive time is a complex task that requires thorough pre-spud planning to identify drilling risks and geological hazards and to develop contingency plans for handling those risks. Building a mechanical earth model during the well planning phase and revising it in real time has proven to be extremely valuable in delivering complex wells safely while minimizing unplanned well construction costs. Monitoring and revising the model while drilling requires geomechanics expertise, teamwork, data management and excellent communications among service companies and their client. This paper defines a mechanical earth model, explains why it is important, how it is developed and how it is applied to well construction and field development. We will discuss sources of information and the multi-disciplinary team approach required to: generate, revise and maintain an earth model. Three examples of the application of the earth model concept are discussed. Introduction More of today's well construction and field development projects are both technically and economically challenging. Understanding the geomechanics of well construction is becoming increasing important in order to drill technically and economically challenging wells on budget. Wells with hostile pore pressure and fracture gradient profiles require a good pre-drill pore pressure and fracture gradient prediction in order to design a suitable casing program. A casing program designed on a profile significantly less hostile than that encountered may compromise the attainable TD of the well. The cost of materials and rig time spent running extra casing significantly adds to the cost of the well. The risk of taking kicks which can be both costly and dangerous can also be reduced by a more rigorous pre-drill pore pressure prediction coupled with real-time pore pressure analysis from LWD measurements. In the deepwater Gulf of Mexico there are examples of wells which require a good mechanical earth model (MEM) in order to be drilled at all. Despite decades of industry attention, wellbore instability is responsible for many costly stuck pipe incidents. Stuck pipe is responsible for lost BHAs and considerable NPT spent freeing pipe, performing additional wiper trips and hole cleaning. In cases where wellbore stability problems are severe, the economics of developing a field can become challenging, for example the Cusiana field in Colombia, S.A. Other fields where lesser wellbore stability problems may still challenge the field economics are found where the cost of drilling is very high, e.g. the Hibernia field offshore Canada and or fields in the North Sea.