Abstract

Tight gas wells often seem to exhibit linear flow for long periods of time; up to 20 years. The cause of this linear flow is not certain, but may be associated with systems of natural fractures. In a particular tight gas reservoir, some wells exhibits bilinear flow behavior. This bilinear flow lasted for over a year in one well. Cinco-Ley and Samaniego1 introduced the idea of bilinear flow in hydraulic fractured well for constant rate production. Later, Guppy et al2 extended the work for constant pwf production. To best of our knowledge, nothing has been reported explaining the causes of the long-term transient performance and production analysis of tight gas wells. This current work was done to determine how bilinear flow might occur in a tight naturally fractured reservoir.

We simulated a single matrix block, which has two distinct permeabilities; a low permeability matrix is surrounded by a high permeability natural fracture. A "well" produces from the high permeability fracture. Two different production scenarios, constant pwf and constant rate, were simulated.. Different cases were run with different matrix block aspect ratios and fracture conductivity.

We also verified that analytical solution for bilinear flow developed by Cinco-Ley and Samaniego1 and Guppy et al2 applies to all cases. We observed some cases exhibit linear flow following the bilinear flow in some cases. So far, we have simulated bilinear flow only in a single matrix block. It has not been determined yet how these single matrix block results relate to a well in an actual reservoir with many matrix blocks. However, this seems to be a starting point to simulate bilinear flow in naturally fracture reservoirs.

Introduction

Bilinear flow regimes have been discussed and reported in numbers of gas wells and is recognized by a 4 t pressure drop behavior during transient flow. Some conditions causing bilinear flow are: a vertical well between two parallel leaky boundaries due to faulting or sedimentary process, a vertical well near a high conductivity infinite fault, a vertical well with a finite conductivity fracture3,4, a horizontal well in a fractured reservoir with transient dual porosity behavior during the intermediate linear flow period, a horizontal well in a layered with transient dual porosity behavior during the intermediate linear flow period and linear reservoir with transient dual porosity behavior. With all these various causes of long-term bilinear flow misinterpretation of production and pressure data may result.

In this work, we show how bilinear flow relates to flow from a single matrix block. We compare this to the Cinco-Ley and Samaniegol and Guppy, et al2 solutions to validate early solutions and our geometric transformation. A number of cases were run to compare to fracture conductivity. Generally, data changes are only made to fracture half-length and fracture permeability. Finally in the last section we show dimensionless variables used to characterized the behavior of the matrix block drainage.

FIELD OBSERVATIONS

The emphasis will be placed on long-term bilinear flow.Field examples of bilinear flow encountered at Well Ou-79.

This content is only available via PDF.
You can access this article if you purchase or spend a download.