Abstract

A minifrac test is usually performed before a fracture stimulation treatment to calculate formation and fracture properties. Recently the analysis techniques were extended to the after-closure period. The after-closure data are analyzed to calculate formation permeability and reservoir pressure.

Technology developers have hypothesized the existence of either pseudo-radial or linear flow behavior during the after-closure region. Identifying the presence of the flowing regime is an awkward process at best. The roots of the linear flow equations are different from those of the pseudo-radial flow equations. Many tests do not follow either flow regime.

In this paper, we have created a general approach for analysis of after-closure pressure decline data. Because the determination of the flow regime and type of fracture depends only on time and monitored pressure, the analysis may even be performed in real time. The technique determines whether sufficient data have been obtained to perform a reliable analysis. The calculated parameters would be used to update the fracture design and, in turn, for performing the fracture treatment.

The new technique is simpler and more generalized than what currently exists. The technique initially determines whether analyzable data exist. It shows that three flow regimes may dominate the after-closure region, depending on the reservoir properties and residual fracture conductivity. The technique presented not only determines the type of regime, and consequently, the type of residual fracture, it also determines the formation permeability and reservoir pressure.

There is no reason to restrict the application of this test to minifrac test analysis. We believe the approach is also applicable to analysis of data after performing a fracture stimulation treatment. A numerical simulator was used to model the pumping and closure process and to validate the new approach.

The paper also presents a detailed discussion and analysis of several field cases, demonstrating the various flow regimes and, ultimately, the validity of the developed technique.

Introduction

Minifrac analysis has considerably progressed since it was introduced by Nolte.[1] This is especially true during the last few years. The boundaries between conventional fracture diagnostic service and conventional well testing are already blurred. Most of the developed analysis techniques presented so far concentrate on the analysis of the before-closure data.[2-14] Recently, analysis techniques for after-closure data have been introduced.[15]

The various diagnostic techniques include the conventional methods whose goal is to determine closure pressure and leakoff coefficient, and the latest development of the area leading to calculation of reservoir properties such as pressure and permeability. These tests are conducted after closure, and their analyses rely heavily on the conventional well-testing technology. This paper includes new technology that has not yet been presented in the literature.

In addition to conventional well testing, several other specialized tests are performed before, during, or after a hydraulic-fracturing treatment to determine formation and/or fracture parameters. These fracturing tests are performed for the following reasons:

• Better understand the formation physical and mechanical properties.

• Predict formation response during the fracturing process.

• Optimize the fracturing treatment design.

The goal of the tests is to determine many of the various parameters influencing a fracturing treatment such as fracture closure pressure, instantaneous shut-in pressure (ISIP), fracture opening pressure, formation leakoff coefficient during a fracturing treatment, and fracture entry pressure. More modern approaches aim at also obtaining formation permeability and original reservoir pressure. There are three basic fracturing tests:

• Step-rate

• Pump-in/flow-back

• Pump-in/shut-in