Prefracture Injection Surveys: A Necessity for Successful Fracture Treatments
Authors: Bundy Thomas E. 
Published: December 2007

Successful fracture treatments over large intervals are often difficult to obtain. In the Laredo Lobo gas field in Webb and Zapata Counties, TX, successful fracture treatments over large intervals are critical for economic success of the field. Conoco Inc. uses a pre fracture injectivity test along with temperature and gamma ray logs to ensure that the entire completed interval will be treated during the fracture job. If the entire completed interval is not being treated, the logs act as a road map to determine where additional perforations are required.

Introduction

Post fracture evaluation logs run in the Laredo Lobo field show that often only a portion of the completed interval was treated. Successful fracture jobs are difficult because the wells are completed over intervals as large as 61 m (200 ft) that often contain several individual pay sands. A typical log section is shown in Fig. 1. Successful fracture treatments over large intervals are critical for the future of Laredo field, as wells completed over small, easy-to-stimulate intervals are often uneconomical.

Many techniques have been used to treat these large intervals. In the early life of the field, staged treatments with ball sealers were used widely. Currently, the use of ball sealers has been abandoned because post fracture evaluation logs have proved them ineffective. Staged treatments with temporary sand, gel, or mechanical plugs, although effective, are often impractical because of high pressures and small shale intervals that separate the pay sands. The most effective treatments involve the limited-entry technique to divert the fracture fluid across the entire interval. However, the success of the limited entry treatments has been hampered by poor perforating efficiencies and low fracture rates.

The low fracture rates in the Laredo field, which are caused by tubing and pressure restrictions, require that a minimum perforation pressure drop be used in the limited-entry fracture design. To distribute the fracture fluid accurately across the completed interval with a minimum perforation pressure drop, the fracture gradients of the individual zones in the completed interval must be known. In the Laredo field, the fracture gradients of the individual zones cannot be determined until the well is fractured. If the fracture gradients of the individual zones are assumed incorrectly, the fracture job will not treat the entire completed interval.

This paper proposes that a pre fracture injectivity test be run to check assumptions made in the design of a limited-entry fracture treatment. Temperature and gamma ray logs run in conjunction with the test are used to determine whether the entire completed interval is being treated and to find any mechanical problems with the well before an expensive fracture job is pumped.

Theory

Most fracture jobs designed to treat large intervals fail because the fracture fluid is not distributed properly over the entire completed interval. When a particular zone in the completed interval does not receive enough fluid, it screens out early in the fracture job because the necessary fracture width is not created.

The limited-entry technique has been the most effective means to distribute the fracture fluid across the completed interval. In limited-entry treatments, a minimum pressure drop of 2100 kPa (300 psi) is maintained across the perforations to force fluid into all the perforations. Many limited-entry treatments fail because the fracture design includes the assumption that a single fracture will be created across the entire completed interval. In practice, several independent fractures will be created across the completed interval when sufficient barriers separate the individual pay zones.

JPT

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