Application of Risk Analysis Methods to Subsurface Well Collisions
- J.L. Thorogood (BP Exploration) | T.W. Hogg (BP Exploration) | H.S. Williamson (BP Exploration)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1991
- Document Type
- Journal Paper
- 299 - 304
- 1991. Society of Petroleum Engineers
- 3 Production and Well Operations, 1.7 Pressure Management, 1.7.5 Well Control, 7.2.1 Risk, Uncertainty and Risk Assessment, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.1 Well Planning, 1.6.6 Directional Drilling, 1.9.4 Survey Tools, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 1.6 Drilling Operations, 1.10 Drilling Equipment, 6.1 HSSE & Social Responsibility Management
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Subsurface separation criteria have evolved empirically over the years. They still are based largely on untested assumptions about safety factors, comfort values, and survey tool accuracy. A mathematical analysis of the probability of collision combined with a decision tree describing the consequences provides a method of risk evaluation. The mathematics can be simplified under certain special assumptions, allowing key features of the problem to he illustrated. A flow chart of the directional-drilling tolerance setting procedure shows how the methods described can be used in daily well-planning operations.
Formal methods for planning deviated wells, determining safe interwell separations, and executing drilling programs are poorly described in the literature. Basic geometrical calculations are covered in textbooks, but the more detailed procedures for operating on multiwell platforms have evolved gradually over the years and are largely undocumented. Two approaches commonly are used to establish safe well separations. 1. A set of fixed separation guidelines is defined as a function of depth. This method has the major advantage of simplicity. The rules may be empirical or may have been derived from an analysis of survey errors. The principal difficulty with this method is that there is no way to assess whether the values are conservative. 2. Ellipses of uncertainty can be calculated and separation criteria can be based on a minimum allowable distance between ellipses. While appearing to be more "scientific," many uncertainty models are not formally validated, and the use of confidence intervals appears to be quite arbitrary. Consequently, users are again unable to assess whether the predictions are conservative. In the face of the twin pressures of safety and cost-effectiveness, neither procedure allows the planner to balance the sizes of tolerances, costs of surveying, efficiency of drilling, loss of production, and probability of collision against the consequences of a production, and probability of collision against the consequences of a collision. Therefore, there is good justification for developing Procedures that enable engineers to demonstrate the optimum Procedures that enable engineers to demonstrate the optimum operational plan when the consequences of undetected errors have been minimized. This problem has five solution components: (1) a set of formally validated models of instrument behavior; (2) a mathematical estimate of probability of intersection between two wells at a specified separation for a given level of survey uncertainty; (3) a method establish maximum tolerable probability of intersection between two wells; (4) a procedure for defining subsurface tolerances based on the intersection criteria; and (5) a management structure for plan execution at the wellsite. The purpose of this paper is to describe a risk-analysis-based solution to the well-collision problem embodying three new ideas: a method to derive maximum tolerable intersection criteria, calculation of intersection probability between wells, and a method to integrate these solutions into the directional-well planning process.
The process of risk analysis involves three steps: devising an event/outcome tree, quantifying the consequences of different branches, and assessing whether the resulting risks are tolerable. Inspection of the well-intersection problem indicates that the most important considerations are fluids in the well, the flowing characteristics of the well and its pressure regime; the nature of any barriers to the blowout, such as a blowout preventer (BOP) or subsurface safety valve (SSSV); properties of the drilling well, including mud weight and fracture gradient; and probability of ignition of the blowout. The problem may be analyzed by means of an event/outcome tree (Fig. 1 and Tables 1 and 2).
|File Size||550 KB||Number of Pages||6|