A Case Study of Integrating Well-Logs and a Pseudo 3D Multi-Layer Frac Model to Optimize Exploitation of Tight Lenticular Gas Sands
Authors: Kordziel Walter R. , Rowe Wayne , Dolan V.B. , Ritger Scott D. 
Published: December 2007

This paper is a case study describing an integrated services approach to growing a vast, but marginal asset into an extremely successful, long-term exploitation program. The producing horizon is a series of tight lenticular gas sands requiring adequate stimulation to be productive. Past failures to disseminate appropriate data between exploration and completion groups often resulted in non-stimulated or under-stimulated pay sands. Facing increasingly difficult economic realities a more synergistic methodology was necessary to economically optimize the contribution from each pay sand. Well-logs and mud-logs are evaluated to group each sand into zones for staged hydraulic fracturing. Stress data from dipole shear imaging tools are combined with leakoff data acquired from mini-frac techniques to individually design the treatment for each stage using a multi-layered pseudo-3D frac model. Stimulation effectiveness is evaluated using real time treatment pressure matching, multiple isotope tracers and production log profiles. The integrated team continually evaluates the petrophysical interpretation, stimulation effectiveness and execution processes to eliminate redundancies, improve well performance and lower exploitation costs. This methodology is accelerating production, increasing reserves and lowering well costs over previous programs allowing the program to continue in spite of falling gas prices.

Introduction

In 1993 Amoco held a large acreage block of undeveloped leaseholds in the Wamsutter Field located in the Greater Green River Basin of south central Wyoming, Fig. 1. These undeveloped leases begin expiring in 1998 if they are not drilled. The producing horizon is a series of low permeability lenticular gas sands requiring hydraulic stimulation. High well costs, relatively poor prospect quality and low gas price forecasts render most of the potential locations noneconomic. Fig. 2 demonstrates that with original well productivity rates and associated total well costs only 18% of the acreage would be economic to develop.

Amoco will lose a valuable resource unless productivity is increased and well costs significantly reduced. Accomplishing this required a radical change in business approach. Amoco responded by assembling a multi-disciplined, multi-company alliance team to drill as many locations as economically feasible. Each aspect of the operation, from permitting through first sales, has been analyzed and re-engineered. This paper details the results achieved in the well completions portion of the operation by taking advantage of this synergistic, total-team approach.

A completion team including geologists, production and reservoir engineers, wireline specialists and pumping services specialists was formed. The team evaluates wellogs and mudlogs to identify which of the many lenticular sands are productive. These sands are grouped into zones for staged hydraulic fracturing. Stress data from dipole shear imaging tools are combined with leakoff data acquired from mini-frac techniques to individually design the treatment for each stage using a 2D multi-layer and pseudo-3D frac models. All stages are fraced in a single, highly coordinated operation.

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