This paper presents a case study review of the performance benefits of a 66 well Coal Bed Methane (CBM) development program whereby cased-hole (CH) logging was introduced as a substitute for open-hole (OH) logging. This paper demonstrates how the reduction in OH logging requirements improved time and cost performance and reduced risk exposure while maintaining suitable data quality.
The approach was based on utilizing existing offset well data and three-dimensional (3D) seismic data to substitute OH density logging for CH density logging. Therefore reducing critical path time on the drilling rig and risk exposure with wireline logging tools in an OH environment. A qualitative and quantitative approach to review rig performance and cost benefits was subsequently undertaken.
The density log was of key importance as it is used to define net coal thickness in CBM reservoirs. Current CH density logging typically results in the reduction of log resolution and accuracy which limits the ability to resolve thinner coal seams. The loss of coal definition has the potential to result in the under-estimation of net coal for reserves purposes, impact static modelling and possibly negatively impact fracture stimulation decisions.
The 66 well development program provided a suitable data set to quantify the performance benefits of a combined CH and OH evaluation program. The transition to CH density logs removes online evaluation activities during the drilling operation. This provides a direct time and associated spread cost saving. In addition, unsuccessful OH logs were replaced with CH density logs when hole conditions presented higher likelihood of unproductive and high risk operations (tight hole/fishing). The campaigning of CH logging operations further optimized both wireline and operating company resources and time. The CH density logs provided suitable data to adequately identify coal targets in order to stimulate and complete the wellbore. The reduction in log resolution and coal identification was overcome by developing 'optimized' density cut-offs based on a calibration study with OH density wireline data.
This case study supports the use of CH density logs in lieu of OH density logs where feasible for CBM reservoirs. It demonstrates the operational optimizations and cost savings by conducting wireline logging operations offline while also reducing risk exposure to down hole issues.