Abstract
Unconventional reservoirs such as the Vaca Muerta formation in Argentina continue to be an important source of oil and gas, and represent a vital source of energy for specific segments of the global market. Achieving technical and financial objectives when developing such reservoirs requires operators to manage operational costs while still obtaining sufficient geological, petrophysical, and geomechanical information to successfully evaluate and develop the reservoir. In a recent Vaca Muerta exploration well, LWD resistivity and azimuthal sonic measurements were combined with advanced mud gas analysis of C1 to C8, toluene, and benzene gas components. These two data sources, acquired while drilling, were used in an integrated petrophysical analysis which characterized the stratigraphic variation of key reservoir parameters and geomechanical properties to identify sweet spots for further horizontal well development.
Geomechanical analysis from azimuthally oriented LWD slowness measurements revealed stressinduced anisotropy by identifying the splitting of shear slowness around the wellbore. The comparison with wireline sonic and micro-imaging tools in the same well confirmed the relatively isotropic acoustic response (2-3 % anisotropy) in the upper section and a more anisotropic acoustic response (7-9 % anisotropy) in the lower section of the Vaca Muerta. Additional mechanical properties such as elastic moduli were calculated to evaluate fracability, and the unconfined compressive strength (UCS) was correlated to the mechanical specific energy (MSE) from drilling mechanics measurements to monitor drilling efficiency.
Mud gas ratio analysis revealed variations in reservoir thermal maturity, identifying wet gas and dry gas intervals, while LWD resistivity and sonic data were used to compute TOC (using the Passey overlay method), porosity, and water saturation. The key reservoir quality parameters (TOC, porosity, water saturation, thermal maturity, and clay volume) were then combined with geomechanical properties (Poisson’s ratio, Young’s modulus, and UCS).The resulting interpretation provided a continuous evaluation of the entire Vaca Muerta sequence, facilitating zonation and selection of potential landing points for horizontal development wells.
This comprehensive evaluation from the integration of the LWD azimuthal sonic and advanced mud gas analysis demonstrated a cost-effective solution to characterize the formation which enhances real-time decision making, and provides the basis for optimizing well placement and completion strategies in the Vaca Muerta formation.
