Evaluation Of Apparent Fracture Lengths Indicated From Transient Tests
Authors: Elbel Jack , Ayoub Joseph 
Published: March 2010

Pressure transient analysis of post-fracture buildup data often implies short, effective fracture penetration, even after large volumes of proppant have been injected. This creates problems when attempting to enhance the economics by improving subsequent fracture treatments.

This paper illustrates how this problem can be resolved by applying a correct interpretation methodology and by integrating available data from other disciplines such as hydraulic fracturing, and log and core analyses. Field examples are used to explain the causes of the inconsistencies and to depict how the integration process leads to more consistent answers.

Introduction

Pressure transient analysis is commonly used to evaluate the effectiveness of fracturing treatments. A post-fracture well test is conducted to estimate me fracture penetration and conductivity to allow the prediction of the expected well performance or to consider the design changes in subsequent fracture treatments. However, well test analysis often indicates short fracture lengths even after large volumes of fluid and proppant have been injected into the formation. This paper discusses the possible reasons for such an analysis, and illustrates the importance of integrating the well test with the production data, fracturing data, and log and core information to achieve a better evaluation. The proper identification of the models used in the evaluation is essential to enhance the economics of a stimulation project.

Theoretical Considerations

The integration of the available data will alleviate the uniqueness problem inherent to any interpretation process by reducing the number of alternative solutions(1,2), Stand-alone well test data may infer short fractures when compared to the fracture design. This could be caused by the following reasons.

(A) Design shortcomings due to inappropriate fracturing geometry models or design parameters.Short propped length is actually the condition because of excessive height growth or tip screenout(3,4)Proppant permeability is damaged or plugged due to insufficient polymer degradation(5,6).There is incomplete proppant height over the net pay due to proppant settling during c1osure(7).There is little or no proppant penetration in some zones during multizone fracturing(8).

(B) Well-test-analysis shortcomings due to the inappropriate identification of the correct reservoir/well models or the misapplication of analysis technique.Semilog analysis with insufficient shut-in time(9).Insufficient production or cleanup of fracture fluid prior to the test(10–12).Presence of significant fracture face damage(13,14).Use of homogeneous model when layers with high permeability contrast exist(15).Use of a single-layer model in a layered reservoir with different fracture penetrations in each layer(l6).Assumption of uniform conductivity when fracture conductivity varies(17).Significant non-Darcy flow in the fracture(l8).Unfavourable permeability anisotropy (19).Unequal fracture wing penetration (ratio less than 0.3) (17).Vertical fracture in a deviated wellbore(20).

The fact that various explanations can be made for an apparent shorter fracture than predicted by the geometry model creates a problem when trying to improve the results of subsequent fracture treatments. A