COLT Case Study - Texas

A combination of UBCTD and new formation evaluation technology improves real-time decision making on location.

Underbalanced Coiled Tubing Drilling (UBCTD) and RockSense were utilised in a two well re-entry campaign in the Texas Panhandle. The project was part of a wider campaign to revitalise a field that had been producing for many years by intercepting fractures and accessing untapped reserves to boost production.

Objectives & Challenges

The two wells under consideration were primarily gas producers, with condensate and water also being produced. UBCTD was seen as the best way to drill the reservoir underbalanced because it provided continuous stable underbalanced bottom hole pressure, as well as fast, safe and effective operation in two phase flow. The critical requirement was to stay in AFE, with a related aim of showing that UBCTD could be carried out cost effectively in the US and generate a significant ROI.

The candidate wells needed to be evaluated for stability in the overburden immediately above the pay zone. Due to a history of wells watering out in the area, water also needed to be assessed.

Pre-Job Planning

Pre-well planning showed that only one candidate was able to be drilled underbalanced with an acceptable level of risk. All other wells had substantial amounts of shale interbedded with the limestone in the build section and therefore there was a high risk of shale instability. It was for this reason it was decided that the second well would have an intermediate section drilled and cased by a conventional drilling rig.

The outcome of the planning was a detailed set of written procedures outlining exactly how the project was to be delivered. It also enabled contingency planning to deal with wellbore stability in the build section, and water in the horizontal, to be carried out. Such contingencies included an expandable liner being kept on standby for wellbore stability issues and inflatable plugs being available in case of hitting water.

In the early stages of the project, it was decided to take advantage of RockSense, AnTech’s proprietary synthetic porosity service, which has the ability to distinguish between different formations by combining downhole and surface data in real time to give an at-bit measurement of porosity. The technology offers two considerable advantages over traditional geosteering techniques. First, because wired telemetry has a high data rate, multiple measurements can be made for every foot drilled and operators can gain inch-level resolution. Second, because the data delivered are representative of conditions at the bit – not 20-25ft behind it – the driller can deliver an optimally placed wellbore, with more feet drilled in the target zone. Using RockSense would mean real time decision making would be possible in this project.  

Solution

In well one the original tubing was pulled as planned, a cement plug was set to isolate the perforations and a casing exit carried out by a workover unit was created. The hole was left overbalanced with lease water in the hole and shut in on closed valves. The UBCTD package was then mobilised to the site and rigged up with the well treated as live.

The BHA was run to immediately above the window and the circulation valve in the BHA was opened. The well was then displaced to nitrogen to underbalance the well. The circulation valve is a multi-action electrically operated valve and so it could then be closed for drilling. In order to pass through the window, the gyro integrated sensor was used. It provides gyro-while-drilling capabilities and was used to take a survey and orient the bent housing motor to pass through the window. The technology is incorporated in the BHA in order to assist with passing through windows without getting stuck and for dealing with inaccurate magnetic surveys when close to the existing casing.

Unfortunately, there was difficulty in passing through the window due to an obstruction and the drilling BHA was pulled out of hole after several unsuccessful attempts to pass. To overcome the issue, a milling BHA was made up of a lead mill and a string mill. This was run beneath the 3 1/8” positive displacement motor. The window was dressed using the milling assembly and on the subsequent run in hole with the drilling BHA there was no issue with passing through the window.

The build section was then drilled with dogleg severities of between 25-30 degrees per 100ft. Once the build section was landed, the BHA’s continuously rotating orienter ensured it was possible to continue drilling straight on the horizontal tangent section, even with the large motor bend setting.

The horizontal section was drilled with a hole size of 4½”. By combining RockSense (Figure 1) and production data, several productive gas fractures were intersected. However, water bearing fractures were also intersected.

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In well one the original tubing was pulled as planned, a cement plug was set to isolate the perforations and a casing exit carried out by a workover unit was created. The hole was left overbalanced with lease water in the hole and shut in on closed valves. The UBCTD package was then mobilised to the site and rigged up with the well treated as live.

The BHA was run to immediately above the window and the circulation valve in the BHA was opened. The well was then displaced to nitrogen to underbalance the well. The circulation valve is a multi-action electrically operated valve and so it could then be closed for drilling. In order to pass through the window, the gyro integrated sensor was used. It provides gyro-while-drilling capabilities and was used to take a survey and orient the bent housing motor to pass through the window. The technology is incorporated in the BHA in order to assist with passing through windows without getting stuck and for dealing with inaccurate magnetic surveys when close to the existing casing.

Unfortunately, there was difficulty in passing through the window due to an obstruction and the drilling BHA was pulled out of hole after several unsuccessful attempts to pass. To overcome the issue, a milling BHA was made up of a lead mill and a string mill. This was run beneath the 3 1/8” positive displacement motor. The window was dressed using the milling assembly and on the subsequent run in hole with the drilling BHA there was no issue with passing through the window.

The build section was then drilled with dogleg severities of between 25-30 degrees per 100ft. Once the build section was landed, the BHA’s continuously rotating orienter ensured it was possible to continue drilling straight on the horizontal tangent section, even with the large motor bend setting.

The horizontal section was drilled with a hole size of 4½”. By combining RockSense (Figure 1) and production data, several productive gas fractures were intersected. However, water bearing fractures were also intersected.

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