Despite the dramatic growth of exploration and appraisal activities for coalbed methane (CBM) wells in the Bowen basin, there is still a lack of understanding on the impacts of geomechanics toward well planning, drilling issues caused by borehole stability, and production performance. From the regional core rock mechanics analysis performed so far, we have observed that coals are generally weak and are prone to failure that can lead to significant consequences in drilling and completion for CBM production wells. The geomechanics issues become prominent, especially for long reach and complex multibranched laterals wells, as rock mechanical properties vary along the coalseam target. Proper characterization of rock mechanical properties, especially the unconfined compressive strength (UCS), is essential in wellbore stability analysis.
In this paper, we focus on geomechanics laboratory core analysis techniques to characterize UCS, challenges in UCS core testing on coal samples, and a case study which compares results from five different types of UCS testing on coal and siltstone samples: conventional uniaxial compression, pseudo-UCS, single stage triaxial, multi-stage triaxial, and scratch.
Wellbore stability issues have significant impact in drilling and completing CBM wells in the Bowen basin, especially in long-reach and complex multi-branched lateral wells. Geomechanics evaluation can be performed to mitigate wellbore stability issues by optimizing the mud weight required to keep a stable borehole (Puspitasari et. al., 2014). Done correctly, this type of evaluation has the potential of preventing costly short- and long-term drilling and completion problems.
Proper characterization of rock mechanical properties is essential in wellbore stability analysis. The unconfined compressive strength (UCS) is the most important rock strength parameter to describe stability at the borehole wall (Fjaer et al., 2008). UCS is defined as the strength of a rock or soil sample when crushed in one direction (uniaxial) without lateral restraint (Allaby & Allaby, 2014). A continuous UCS profile along a wellbore is typically estimated using mathematical and empirical equations that uses well logs (such as sonic, density, and GR log) and/or interpreted petrophysical parameters (such as volume of clay, total porosity, and effective porosity). To ensure that the log-derived UCS is representative of the actual rock strength, the equations must be calibrated with direct measurement from laboratory core tests.
Several types of laboratory test can be performed to characterize rock strength. The tests are typically done on cylindrical samples (core plugs) obtained at the depth of interest. The size of the plug is typically 1.5 in. in diameter and 3 in. long. For small grain-size rock, such as shales, smaller plug sizes can be used. The small shale plug size is ideally 1 in. in diameter and 2 in. long, but it can also be as little as 0.75 in. in diameter and 1.5 in. long. Table 1 compares of some of the most common tests used to measure UCS in the industry.