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  • Writer's pictureAli salmachi

Drilling data of deep coal seams of the Cooper Basin: Analysis and lessons learned

Updated: Mar 23, 2018

In this short article, I explain how drilling data analysis can help reservoir engineers in finding answers to two questions:

1) Does wellbore communication exist among the nearby wells?

2) Is the hydraulic fracture confined in the target zone? or has propagated into neighboring formations?

I use the drilling data obtained from mud logging units of the wells penetrated target deep coal seams of the Cooper Basin in the Weena trough (Klebb area) to answer those two important questions. Figure 1 is the map of the Cooper Basin in South Australia showing the Klebb area located in the Weena trough.

Figure 1: Map of the Cooper Basin.

The mud logging unit was first introduced commercially to the oil and gas industry in 1939. Since then, mud logging functions and capabilities have significantly evolved to perform functions such as core analysis, drilling data acquisition, downhole measurement while drilling and directional drilling measurement. Gas detection, as well as drilling information such as rate of penetration (ROP), weight on bit (WOB) and rotary speed (N) are recorded in mud logs.

We constructed a drillability index (DI) by normalizing drilling rate using drilling factors including rotary speed, weight on bit, bit diameter, drilling fluid density and hydraulic impact force (HIF) at the bit face. The resulting drillability index allows estimating coal rock strength in the study wells (Klebb 1, Klebb 2 and Klebb 3). We also collect gas show in target coal intervals. Figure 2 shows drilling factors, gas show and calculated drillability index for one of the study wells (Klebb 2). Note that drillability indexes were only calculated at the coal intervals.

Figure 2: Drillability and drilling information for Klebb 2.

The high drillability index for coal intervals at depths greater than 1900m are attributed to existence of a cleat/fracture network. A fracture/cleat system can substantially reduces coal strength and as a result of that drillability index increases. Note that drillability index is sensitive to a number of factors and hence, it should be studied along with other reservoir information to yield conclusive results.

Figure 3 demonstrates gas show results for a number of coal intervals including target coal seams that are hydraulically fractured in the Klebb wells.

Figure 3: gas show results for a number of coal intervals in the Klebb wells.

Klebb 1, 2 and 3 were brought to production in a chronological order to dewater target coal seams. Gas show trends of coal seams are consistent among the three wells down to about 2000 m and then start separating from each other at coal intervals that are hydraulically fractured. The maximum gas show in Klebb 1 does not exceed 400 units, which is typical at this depth. However, gas shows in Klebb 2 and 3 are significantly higher, with maximum gas shows of 2035 and 2798 units, respectively. The significantly higher gas shows in Klebb 2 and 3 are attributed to wellbore connectivity among Klebb wells due to existence of a natural fracture network and hydraulic fracturing. Dewatering in Klebb 1 depressurizes the reservoir and facilitates gas desorption and diffusion into the fracture network and as a result, higher gas shows are observed while drilling Klebb 2 and 3. Wellbore connectivity is useful in coal seam gas reservoirs facilitating gas desorption and production.

We can also utilize the drilling data to answer to the second question " Is the hydraulic fracture confined in the target zone? "

A coal interval, separated by a shale formation from the upper and lower target seams in Klebb wells, is shown in Figure 3 as the reference coal interval. This coal interval may be used as a reference to investigate confinement of the hydraulic fracture height within target formations. Drillability index of the reference coal interval is high, similar to the upper and lower target seams (see Figure 2) suggesting that coal seam is permeable. This coal seam is not hydraulically fractured and is not a part of the well completion. Any hydraulic connectivity, established naturally (e.g. existing faults/fractures) or artificially created by hydraulic fracturing between the reference coal interval and the target coal seams maybe identified by monitoring gas show readings of the reference coal interval. The reference coal interval acts like a “virtual gauge” monitoring pore pressure that regulated gas desorption from the coal matrix into the cleat network. The gas show readings of the reference coal interval remain approximately equal in Klebb 1, 2 and 3 unlike the gas show reading in the upper and lower coal sections completed by hydraulic fracturing. Since gas show readings remain constant in the reference seam, it can be concluded that there is no hydraulic connectivity (naturally or artificially created) between fractured seams and the reference seam. This reference coal interval provided an opportunity to conduct a hydrology assessment in the Klebb area to ensure the hydraulic fracture has not propagated into the upper formations.

This article shows the importance of drilling data and how data analytic can help reservoir evaluation in deep coal seams of the Cooper Basin.

Full article will be published in APPEA journal in May 2018.

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