Estimating Geo-Mechanical Strength of Reservoir Rocks from Well Logs for Safety Limits in Sand-Free Production

Abstract

Hydrocarbon exploration and exploitation does not only require the knowledge of hydrocarbon in-place, however, mechanical competency of the reservoir rock must also be known. The direct method of inferring S- and P-wave velocities from seismic data usually has limitation of poor resolution because of the uncertainty in seismic inversion which may also affect other derivatives.

An analytical method is presented with the possibility of predicting shear wave velocity from wireline log data where S-wave sonic logs do not exist. By estimating S-wave velocity, formation geo-mechanical properties can be calculated using P-wave sonic and density logs with appropriate equations. Elastic constants such as Poisson Ratio, Young’s, Shear and Bulk moduli which are the parameters for characterizing rock mechanical properties were estimated and used to predict the mechanical competency of the formation for hydrocarbon exploration. Well planning demands knowledge of these geo-mechanical properties which can be used to estimate the pressures required to initiate a fracture into a formation for the safety of the personnel and equipment, in particular minimizing the associated risks.

In this paper, firstly we investigate the possibility to predict the shear velocity from well logs, and then Elastic moduli calculated from the log data can therefore be used effectively in predicting safety limits in sand-free production from friable sandstones.

The results of this study shows that the combined modulus of strength (K) and the shear modulus (S) to compressibility (c) ratio (S/c) for the formation are relatively low. The average value of K is lower than the threshold value indicating the minimum value at which fluids may be produced safely at any rate but falls within the range which generally represents a condition in which the problem of sand production should not arise below a certain optimum flow rate. Average value of S/c ratio is lower but tends towards acceptable range.

Keywords: Mechanical competency, predicting shear wave velocity, elastic constants, well planning, combined modulus of strength, shear modulus to compressibility ratio.