Field development of a reservoir generally involves the drilling of extended reach wells for maximum hydrocarbon exposure and productivity. In a conventional reservoir, porous and permeable formations support fluid movement under the influence of a drive mechanism (generally water or gas) and wells are normally positioned high up in the structure for maximum recovery. In contrast, an unconventional reservoir is typically heterogeneous with hydrocarbons trapped within diagenetic horizons that may lie anywhere within the structure.
The challenge in an unconventional reservoir is to identify a productive zone where heterogeneity exists at a microscale which is beyond the resolution and visibility of traditional LWD and wireline sensors. These properties could be identified directly from core samples in a laboratory environment but repetitive coring in an extended reach well is unfeasible. However the advent of portable micro-scale measurements (x-ray diffraction XRD, x-ray fluorescence XRF, pyrolysis, scanning electron microscopy SEM and nuclear magnetic resonance NMR, total organic content) has made it possible to obtain core quality measurements from drilled cuttings at the wellsite.
By combining LWD and advanced cuttings analysis at the wellsite a unique solution is provided for the unconventional reservoir challenge. LWD provides larger scale stratigraphic positioning, boundary monitoring and reservoir model updating, while cuttings analysis provides confirmation that the wellbore is being drilled and tracked within the correct target layer.
In the described example we demonstrate the practical application of this approach in a limestone reservoir with considerable heterogeneity. Comprehensive data from the pilot well (including core) was combined to generate an earth model. Detailed core analysis including; elemental, chemostratigraphy, mineralogy, petrology, total organic carbon, density, porosity and permeability, was then used to identify the target zone.
LWD azimuthal resistivity was used to steer and land the wellbore within the reservoir and then to monitor distance-to-bed boundary for the cap rock. With the reservoir formation possessing low natural radioactivity and low contrast resistivity, advanced cuttings analysis enabled steering decisions to be made based on microscale variables including elemental abundance and elemental ratios such as As:Cr, Mg:Mn, SiO2:Al2O3, and Ga:Rb. The well was successfully drilled and positioned within the target layer for maximum recovery.
Restricted fluid mobility and constrained drive mechanisms within unconventional reservoirs result in recovery factors being much lower than for a conventional case. Secondary or tertiary recovery suddenly becomes comparable to the primary recovery. Micro-scale information required for an enhanced recovery strategy is already provided by the advanced cuttings analysis acquired during the drilling of the initial well.