Abstract

In this paper we present a new multiwell reservoir solution and an associated analysis methodology to analyze single well performance data in a multiwell reservoir system. The key to this approach is the use of FIELD cumulative production data and individual well flowrate and pressure data. Our new solution and analysis methodology "couples" the single well and multiwell reservoir models — and permits the estimation of total reservoir volume and flow properties within the drainage area of an individual well — where the analysis is performed using a SINGLE WELL reservoir model (type curve). This "multiwell analysis" using a "single well model" is made possible by a "coupling" of the single well and multi-well solutions based on a total material balance of the system. The data required for this approach are readily available in practice: basic reservoir properties, fluid properties, well completion data, and well rate (and pressure) data and cumulative production data for the entire field.

Currently, ALL existing decline type curve analyses assume a single well in closed system (or single well with constant pressure or prescribed influx at outer boundary). In many cases a well produces in association with other wells in the same reservoir — and unless all wells are produced at the same constant rate or the same constant bottomhole flowing pressure, non-uniform drainage systems will form during boundary-dominated flow conditions.

Further, it is well established that new wells "steal" reserves from older wells, and this behavior is commonly observed in the production behavior. Our new approach accounts for the ENTIRE production history of the well and the reservoir, and eliminates the influence of "well interference" effects. This approach provides much better estimates of the in-place fluids in a multiwell system, AND the methodology also provides a consistent and straightforward analysis of production data where "well interference" effects are observed.

This work provides the following deliverables:

  1. A new solution for a multiwell reservoir where the formu-lation of this solution yields a simplified form for an arbi-trary (individual) well during boundary-dominated flow conditions.

  2. A complete analysis methodology for oil and gas reservoir systems based on conventional production data (on a per well basis) as well as the cumulative production of the entire field.

  3. A systematic validation of this approach using a numerical reservoir simulator for cases of homogeneous, regionally heterogeneous, and randomly heterogeneous reservoirs.

  4. An application to a large gas field (Arun Field, Indonesia). This analysis provides very consistent estimates of in-place fluids and reservoir properties. All of these analyses (simulated and field data) clearly demonstrate that the effects of "well interference" on individual wells have been eliminated as a result of this analysis methodology.

Introduction

The single well model has been widely used to forecast the production decline of wells-reservoir systems. Although the analytical solutions for single well in circular reservoir came as early as in 1934,1 this effort was pioneered by Arps2 who presented a suite of (empirical) exponential and hyperbolic models for this purpose.

Fetkovich3 (1973) presented theoretical basis for Arps's pro-duction decline models using the pseudosteady-state flow equation. He also developed decline type curves that not only permit us to forecast well performance, but also allow us to estimate reservoir properties (i.e., flow capacity – kh) as well as oil-in-place. This classic work by Fetkovich laid the foun-dation for all of the work that followed regarding decline type curves.

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