Driven by their desire to lower costs and improve efficiency while measuring the production potential of unconventional reservoirs, operators continue to rely on new evaluation technologies.

The application of Weatherford’s Reservoir Evaluation System (RES) in “mini-fracing” has been covered here before (see the February 2013 issue of E&P). With an extremely high flow rate, the RES also is the industry’s first commercially available pumpout formation tester with a uniform 4.5-in. outer diameter, a feature that minimizes the risk of sticking and thereby limits operator exposure to nonproductive time.

Delving deep down under

The development of Australia’s coalbed methane (CBM) reservoirs, also known as coalseam gas reservoirs, has grown rapidly over the past decade. And with proved and probable methane reserves of roughly 32 Tcf, the Australian state of Queensland is experiencing a major share of the E&P activity.

In addition, the demand for domestic energy in eastern Australia has soared over the last decade, while the industry has been able to prove that coalseam gas is both an economically viable and environmentally sound option.

Meeting this demand in ways that are both profitable and sustainable in the long term is forcing operators to find more reliable and quantitative evaluation methods for these formations.

An important parameter is the formation’s cleat density. Cleats, which are joints confined within the coal seam, impart permeability to the seam. A high cleat density is a prerequisite for profitable exploitation of coalseam methane, and the ability to measure both the density and geometrical orientation of the cleats is essential to drilling and completions planning.

System trial

An operator in Queensland expressed interest in trialing the new evaluation system after several conventional techniques failed to deliver the level of detailed information required on the extent of the cleat network within the coal seam.

For example, acoustic techniques, which measure velocities of compressional and shear waves along with the difference in shear velocity in the direction orthogonal to the wellbore direction, can be used to gain an understanding of the cleat direction and possible permeability. Electric imaging tools also are widely used to capture a range of reservoir features, including dip and fracture detection, sedimentary features, facies analysis, and cleat counts in coal measures.

While both methods provide some estimate of cleat density and allow the operator to glean additional petrophysical data from the reservoir, they provide this information only in the immediate vicinity of the wellbore. A permeability estimate for the entire seam (in zones remote from the wellbore) was not possible with these techniques.

Drillstem tests (DSTs) typically represent the other end of the evaluation spectrum and provide a means of isolating and testing the pressure, permeability, and productive capacity of the formation. During this test, the coal measure is isolated from the rest of the wellbore by inflatable packers above and below the section. A dewatering procedure is performed to reduce the pressure at the coal measure face, which causes fluids to flow out of the packed-off formation and allows the DST to measure parameters such as the volume and flow rate of the fluid with a high degree of accuracy. Quantitative volume and flow rate measurements are critical to accurate formation evaluation.

The company’s Flow Rate Tester (FRT) was used successfully for several years as an alternative by the operator and helped accurately evaluate the coalseam properties with an overall cost that was significantly lower than DSTs.

However, the RES tool offered an even more viable and cost-effective alternative to DSTs by providing an accurate and quantitative measurement of volumes at a lower price point. And, compared to the FRT, the RES offered better flow rate control as well as the capability to pump borehole fluid into the formation, which was not possible with the FRT.

The operator decided to run the RES tool in a well to test its effectiveness, which marked the first deployment of the tool in Australia. Local Weatherford engineers were joined by colleagues from Houston and Canada to help deploy the tool in the field and analyze the results.

Comparing concepts

Several planning sessions between the operator and service company engineers helped identify the right well for a test, which included coal seams of different permeabilities at different depths to gauge the performance of the new evaluation tool against the performance of the more conventional FRT. The test began by performing conventional tests with the FRT tools over all the identified seams in the well. Then, upon discussion with the operator’s personnel, the new tool was lowered to the selected low-permeability seam where the injection (falloff) test had been conducted.

The RES tool was deployed via wireline and set in place using a variable spaced packer system. This system has a customizable spacing from 0.5 m to 5 m (1.6 ft to 16 ft) to adapt to the needed reservoir thickness in this particular scenario. This adjustability allowed the operator to target the key areas of interest in the reservoir to better understand production behavior.

The tool’s pump also provided significant benefits compared to those used for conventional DSTs and FRTs. The pump’s flow rate can be adjusted in a wider range, and its flow throughput is nearly double that of conventional FRT pumps. The accuracy of the volume and flow rate measurements of the RES is far superior to that of the DST and better than that of the FRT tester.

In addition, the flow direction of the pump can be reversed, making it suitable for both injection and drawdown tests. Previous generations of formation testers did not have this flexibility in flow direction, which limited them to drawdown testing only.

Field test objectives

Once the seal was verified, formation pressure evaluation began by conducting an injection test in which pressurized fluid was pumped into the well, and the breaking pressure of the rock was recorded. The field test achieved its objectives of first demonstrating the new tool’s ability to successfully reproduce the evaluation results from a conventional tester in a higher permeability section of the wellbore followed by running a successful injection test in the lower depth, tighter permeability section of the well.

The field trial also confirmed the efficiency gains achievable with the tool. In a 24-hour period it was able to test five to six different coal seams in the same well using the RES tool, while a DST would be able to evaluate only one seam in the same time period.

Ultimately, the new tool’s deployment in this well afforded the operator the ability to perform drawdown and injection testing with greater efficiency and accuracy. The higher quality data afforded by the tool, which was acquired in a shorter time period, gave the operator a clearer geological understanding of the coal seams and a better direction for future drilling and production decisions.

Compared to DSTs, the tool saved approximately five days of rig time, equating to a value of approximately US $150,000. With this saving the operator was able to drill more wells in a given timeframe – a major benefit in this region, where rigs are in short supply.