Application of Risk Analysis Methods to Subsurface Well Collisions

Abstract

Summary 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. Introduction 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:a set of formally validated models of instrument behavior;a mathematical estimate of probability of intersection between two wells at a specified separation for a given level of survey uncertainty;a method establish maximum tolerable probability of intersection between two wells;a procedure for defining subsurface tolerances based on the intersection criteria; anda 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. Risk Analysis 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).

A Geometric Approach to Drill Path Collision Avoidance

“…Most existing approaches to collision avoidance work locally on a point of interest within the reference well rather than a particular interval of interest along the reference well [7,8,9,10,11,12,13]. Computation over an interval, as in our proposed geometric approach, is a more meaningful concept.…”

Application of Spatial Analytic Geometry in the Calculation of Adjacent Well Separation Factors

“…At the beginning, the adjacent well distance scan calculations [1][2][3][4][5] is used to evaluate the degree of compliance between actual wellbore trajectory and designed wellbore trajectory and the collision risk between adjacent wells. In 1991, Thorogood J L put forward the separation factor method to assess the collision risk between adjacent wells [6]. In 1996, Brooks A G introduced a method to evaluate the collision possibility between adjacent wells by probabilistic method [7].…”

A New Look at Wellbore Collision Probability

“…In spite of pioneering attempts to express the complex problem in a simple form (Thorogood et al 1990and 1991, Roper and Henly 1996, Brooks and Wilson 1996, Williamson 1998, most operators today still rely on rule-of-thumb methods with little mathematical foundation. For example, the clearance factor or separation factor is widely used as an indicator of collision probability; while this is basically a ratio of well separation to positional uncertainty, there are many different implementations, none of which bears a strong mathematical correlation to collision probability.…”

“…Methods which solve the problem by integration of two or three-dimensional probability density functions have been described by Thorogood et al (1990 and1991), Brooks and Wilson (1996), Roper and Henly (1996), and Tsao et al (1999). While sometimes less restrictive than the one-dimensional integration, these methods do not completely represent the general problem.…”