The Effect of Areal Variations in Fracture Intensity on N.E. British Columbia Gas Well Productivity
Authors: Roberts Bruce E. 
Published: December 2007

Numerical reservoir simulations using representative N.E. British Columbia Baldonnel reservoir properties were conducted to investigate the extent of drainage in structures with a wide contrast in fracture intensity between the crest and flank of the structure. The simulations showed that although wells drilled into a poorly fractured flank may be nonproductive, the flank gas may still be produced from a high productivity well located in the crestal region of the reservoir. The ultimate recovery depends on the value of the economic flow rate limit. This higher than expected gas recovery is attributed to linear flow along the length of the structure. The initial rate decline is governed mainly by the size of the gas volume-in-place within the more productive, highly fractured zone near the crestal well. Flattening of the rate decline curve due to influx from the reservoir flanks occurs later in the life of the reservoir when a significant pressure difference has been established between the crestal and flank regions. Thus gas located in the poorer productivity structure flanks should not be neglected when evaluating reserves.

Introduction

Wells which encounter natural fractures are often able to yield high flow rates from low permeability rocks. Where fractures have been formed due to folding, the fracture intensity is usually greatest at or near the crest of the structure, but often diminishes toward the back limb. Thus wells are typically targeted for the more highly fractured, and thus more productive, crestal positions.

At issue in this paper is the degree of drainage from wells placed on the structure crest. Whereas these wells should effectively drain the crest of the structure, the drainage of the less fractured flanks of the reservoir is generally less certain. Often, little is known about the recovery efficiency and rate decline trends in reservoirs with these areal variations in fracture intensity.

This study focuses on the drainage of the Upper Triassic Pardonet and Baldonnel formations which are the main reservoirs within the N.E. British Columbia Monkman Pass development area shown in Fig. 1. This play area encompasses a number of gas pools in a northwest trending area approximately 20 km by 100 km [12 by 60 mi] in length. The wells produce dry, sour gas often at rates of the order of 1200 - 2000 × 103 m3/d [40-70 MMcf/D]. Individual pools are estimated to contain between 1 and 9 bcm (billion cubic meters) [35-320 bcf] IGIP (initial gas-in-place) with the average about 2 BCM [70 bcf].

The remote location, mountainous relief, and high acid content (10-40%) result in high capital and operating costs for development of gas pools in N.E. British Columbia. Due to these high costs, typically only 1 or 2 wells per structure can be supported. Given this wide well spacing, the issue of effective drainage of the structure becomes a key factor in drilling and tie-in decisions.

Analysis of historical production to infer extent of drainage is difficult due to long shut-in periods or constrained production (due to market demands), frequent workovers and changes in well tubing head pressures. Therefore in this study, various prototype reservoir simulation models based on representative Baldonnel formation properties were developed to estimate the degree of reservoir depletion. The effects of reservoir properties on rate decline characteristics were investigated.

Reservoir Description

The reservoirs are thrust faulted anticlines, typically 500 - 1000 m [1650 - 3300 ft] in width, and up to 20 km [12.5 mi] in length. P. 525