Claudia Jane Hackbarth

Estimation of Rock Compressive Strength Using Downhole Weight-on-Bit and Drilling Models

In unconventional gas and tight oil plays, knowledge of the in situ rock mechanical profiles of the reservoir interval is critical in planning horizontal well trajectories and landing zones, placement of perforation clusters along the lateral, and optimal hydraulic fracture stimulation design. In current practice, vertical pilot holes and/or the laterals are logged after drilling, and the sonic and neutron log results are interpreted along with mechanical rock properties measured in the laboratory on core material. However, coring, logging, and core analyses are expensive and time consuming. In addition, as they are typically only performed in a few wells that are assumed to be representative, there is considerable uncertainty in extrapolating results across wide areas with known variability in stratigraphy, faults, thicknesses, hydrocarbon saturations, etc. This paper reports a method for estimating mechanical rock properties and in situ rock mechanical profiles in every well in a development, based on calibration from initial rock core analyses plus drilling data that is routinely acquired. Wellbore friction analysis was coupled with a torque and drag model to estimate in situ unconfined compressive strength (UCS) and Young’s modulus (YM) profiles. The key process steps include: a) Calculate the weight and wellbore friction force of each element of the drill string from bottom to the surface; b) Adjust the hook load (HL) by subtracting the weight of the hook and entire drill string; c) Iteratively compute the friction coefficient to match calculated and observed HL; d) Estimate downhole weight-on-bit (DWOB) by applying a stand pipe pressure correction to the calculated HL and considering potential sliding and abrasiveness; e) Use a rate of penetration (ROP) model developed for polycrystalline diamond compact (PDC) drill bits considering a force balance between a drill bit geometry and formation and a wear function depending upon the formation abrasiveness and bit hydraulics to compute confined compressive strength (CCS). The resulting CCS was correlated to UCS and YM using regression constants obtained from laboratory triaxial test data on whole core. Using examples from horizontal wells in a siltstone play in Alberta, Canada, this manuscript demonstrates a workflow to estimate rock strength from drilling data. The predicted UCS and YM values were compared with log data and potential uncertainties arising out of drilling data are discussed.

Pioneering Shale Gas in China: China Sichuan Shale Gas Joint Cooperation Project

Starting in January 2010, Shell and PetroChina have been evaluating an area of Joint Cooperation in the Sichuan Basin, China with the goal to explore, appraise, and ultimately develop a Shale Gas opportunity. The area of interest covers some 3500 km2 in largely agricultural, hilly terrain. Since project inception, two vertical and three horizontal wells have been drilled and hydraulically fractured, demonstrating the lateral pervasiveness of the Lower Silurian Longmaxi Formation shale and the flow of gas to surface in each of the wells. An extensive data gathering and evaluation programme has been executed. Core and log data, engineering data and fluid samples have been collected and analysed. Microseismic monitoring of a 9-stage fracturing treatment horizontal well using deep and shallow instrument wells has also been trialled with good results. This programme is the most advanced foreign co-operation shale gas project in China to date.