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

Abstract

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.

In situ stress magnitude and rock strength in the Nankai accretionary complex: a novel approach using paired constraints from downhole data in two wells

“…As a result, indirect approaches have been widely used to estimate in situ horizontal stress orientation and magnitude from wellbore failures and from fault or fracture orientations measured in borehole logging data (e.g., Moos and Zoback 1990;Zoback and Healy 1992;Brudy et al 1997;Zoback 2007;Chang et al 2010;Ito et al 2013;Lin et al 2015) or from anelastic strain recovery measurements on core samples (e.g., Amadei and Stephansson 1997; Lin et al 2006;Byrne et al 2009). Similarly, in situ strength must be inferred indirectly from downhole drilling parameters (e.g., Kerkar et al 2014) or defined from laboratory measurements (e.g., Horsrud 2001; Song et al 2011;Chang et al 2006). Laboratory measurements require core samples, are time-consuming, and must be upscaled to apply them in the field.…”