An Approach to Locating New Wells in Heterogeneous, Gas Producing Fields
Authors: Coats Keith H. 
Published: December 2007

A two-dimensional numerical calculation of semi-steady-state flow in a gas field can be used to determine the best possible drilling schedule for developing the remainder of the field. The technique can be used also to estimate gas field performance under alternate spacings, to examine lease-line drainage, and to determine the degree of drainage or depletion of different portions of a field.

Introduction
The decline of average field pressure during the producing life of a gas field causes a corresponding producing life of a gas field causes a corresponding decline in well deliverability. Additional wells are therefore drilled during the producing life to maintain the desired or contractual total field producing rate. Maximizing the economic return from the producing operation requires minimizing the number of wells drilled. Even if a fixed total number of wells are to be drilled (ultimately), the usual discounted cash flow or present-value analysis dictates that the wells be drilled as late as possible in project life.
This paper treats the problem of determining an optimum drilling schedule for remaining field development, starting from an initial (present) time corresponding to an arbitrary degree of depletion and arbitrary number and locations of existing wells. This optimum schedule consists of specified well locations to drill and the time at which each is to be drilled.
Conventionally, additional well requirements for maintenance of field productivity are determined from back-pressure curves that relate well deliverability to the difference between average field pressure and flowing wellbore pressures. The assumptions involved in this approach render it satisfactory for only a homogeneous field with regularly spaced wells having equal producing rates.
The method described here minimizes new well requirements at each successive stage of depletion by selecting optimal locations for additional wells. The method accounts for the effects on well deliverability of reservoir heterogeneity, irregular spacing, and well interference.
Following sections state the problem more fully, describe the conventional and proposed methods of solution, and present example applications. The Appendix describes in detail the mathematics involved in the proposed technique for determining optimal well locations.

The Problem
The development drilling program necessary to meet contractual producing rates from a gas field is characterized by a plot, such as Fig. 1, of cumulative wells drilled vs time or vs remaining gas reserves. Time and gas in place are related through the specified field producing rate. An obvious economic incentive exists to keep this curve as "low" as possible. Thus Curve 2 is preferable to Curve 1 even though the total number of wells drilled over project life is the same in both cases. Curve 3 is preferable to both of the other curves. A lower limit on the number of wans drilled is imposed by the requirement of a specified total field deliverability, which may vary with time. The problem is to meet this required field productivity problem is to meet this required field productivity with as few wells as possible. The degrees of freedom or decision variables in attacking this problem are the locations of new wells.
JPT
P. 549