The Austin Chalk formation has undergone exploitation since the early 1920s. The formation, which lies lithologically just above the Eagle Ford shale, is a vertically fractured carbonate ranging in thickness from 50 to 590 ft (15 to 180 m). In oil and gas fields producing from this formation, such as the Pearsall, Giddings, Brookeland, Master’s Creek, and North Bayou Jack, economic production depends on the distribution and density of natural fractures existing in the reservoir as well as the properties of the matrix blocks. Because production trends are inconsistent throughout these fields, horizontal drilling has evolved to enhance well productivity by intersecting multiple fracture sets.

There also are many vertical wells, producing and inactive, in this formation that require revitalization to continue producing or to improve performance. Recently, an operator installed solid expandable tubular systems in 28 oil and gas production wells completed in the Austin Chalk in multiple Texas counties. These workovers were performed in existing wells that exhibited a variety of problems associated with casing leaks (well integrity), isolating depleted zones, extending casing shoes, switching wells from producer to injectors or vice-versa, and other issues. The objective was to maximize asset lifecycles.

Expandable solutions
Avoiding or reducing risks associated with cross flow is one of the most important objectives in specific zones of the

Using solid expandable tubular technology helps correct cross flow problems. (Image courtesy of Enventure Global Technology)

Using solid expandable tubular technology helps correct cross flow problems. (Image courtesy of Enventure Global Technology)

Austin Chalk. With some zones as thick as 590 ft, problems associated with cross flow were identified in almost 30% of the wells. Most of these wells already had low production rates, and cross flow problems contributed to further diminished production.

The workover solution consisted of isolating depleted zones using a single expandable liner. Zones previously used to inject fluid (rather than produce) were sealed, eliminating the cross flow phenomenon and improving overall production while maximizing final reserves recovery.

Well integrity has been identified as an important issue in some wells associated with the Austin Chalk. Many of these existing wells (producers and injectors) are more than 60 years old, and some have problems related to casing integrity, specifically casing leaks. Because most of the wells are depleted and have low reservoir pressure, it could be inferred that leaking does not represent a problem. Nevertheless, operators are avoiding operational and environmental risks by mitigating these casing integrity issues using different technologies.

Solid expandable technology provides the ability to mechanically seal a casing leak without significantly reducing the ID of the base casing. For example, in a well with a 51?2-in., 17 lb/ft, L-80 casing, installation of a 41?4-in. by 51?2-in., 10.7 lb/ft expandable liner only reduces the ID of the casing from 4.892 in. to 4.210 in.

In most cases, this solution will solves the leak on the first attempt, while cement squeezes typically require two or more squeezes to seal a leak successfully. This is an advantage that reduces deferred production, additional costs, operational problems, and possible environmental issues.

A leak in an artificial lift well using an electric submersible pump also can be mitigated using the technology. While it is necessary to use a workover rig to recover the existing well completion and run the solid expandable tubular into the well, this is a single-trip solution that can be run into the hole in less than 10 hours depending of the length of the expandable system. After the expandable liner is installed, the well is ready for production.

Expandable liner sizes were installed as part of the 28-well workover project to correct specific mechanical problems such as leaking and isolating depleted zones or for adding new zones. Most were 41?4-in. by 51?2-in. systems.

The nomenclature used to classify the expandable system indicates the expandable casing size and base casing. For example, in the case of a 41?4-in. by 51?2- in. system, the size of the expandable system prior to expansion is 41?4 in. and the base casing of the well is 51?2 in.

In most cases, expandable technology was used in selected wells to isolate depleted upper zones of the Austin Chalk prior to perforating the same formation in deeper zones. This strategy was implemented to optimize reservoir recovery.

Combined technologies
Studies of cores recovered from select wells indicated that not all natural fractures encountered in this formation are open and contribute to productivity. According to some interpretations, many fractures are closed or mineralized with calcite crystals lining the fracture walls.

Mineralized fractures could have been restricting the flow in some wells, and the possible lithological behavior could explain the successful results obtained when the same formation was perforated in a different zone after the previously producing zones had been isolated. The campaign resulted in a considerable increase in daily production.

Improvements in perforating technology such as special high-penetration charges could help intercept naturally existing vertical fractures that past perforating techniques were unable to connect to the well. It is possible that these two strategies of changing zones and using new perforating techniques are helping operators in the Austin Chalk formation to improve daily production rates and final recovery of reserves. The process associated with isolating an upper zone of the Austin Chalk and recompleting the same well in a deeper zone can be accomplished using expandable technology.

During the feasibility stage of the project, several technologies for isolating current perforations or correcting leaks were evaluated. Some of the most popular technologies, such as straddle packers and scab liners, were rejected because they significantly reduce the well ID. In the case of the cement squeeze, while it does not reduce the ID of the existing casing, it presents a low probability of obtaining a good seal with the first attempt and often requires two or three cement squeeze jobs to achieve the desired results.

The two solutions for isolating an upper existing producing zone in a well completed in the Austin Chalk are artificial lift completion using a straddle packer and expandable technology. A comparison of the solutions shows that the straddle packer creates a substantial restriction in the ID of the completion, which could create difficulties when trying to run special logs, perforate new zones using special charges, or reduce well productivity. Additionally, this restriction could increase the energy required for the artificial lift system to produce a similar production rate.

Expandable technology is providing new and reliable solutions for workover operations in the Austin Chalk. The benefits realized in selected wells illustrate the potential application of expandable technology in mature fields.