Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol.4, No.20, 2014 38 Estimating Geo-Mechanical Strength of Reservoir Rocks from Well Logs for Safety Limits in Sand-Free Production Dorcas S. Eyinla 1* Michael A. Oladunjoye 2 1.Department of Earth Sciences, Adekunle Ajasin University, Akungba Akoko, Ondo State, Nigeria 2.Department of Geology, University of Ibadan, Ibadan, Oyo State, Nigeria * E-mail of the corresponding author: eyinladorcas@yahoo.com 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. 1. Introduction It has been discovered that in many developed oil fields, only compressional wave velocity may be available through old sonic logs or seismic velocity check shots. For practical purpose such as in amplitude variation with offset (AVO) analysis, seismic modeling, and engineering applications, shear wave velocities and moduli are needed. In these applications, it is important to extract, either empirically or theoretically, the needed shear wave velocities or moduli from available compressional velocities or moduli (Wang, 2000). P-wave velocity (Vp) and S-wave velocity (Vs) show a linear correlation in water saturated sandstones as Vs = 0.79Vp – 0.79 (Han, 2004). Castagna (1985) proposed a method for shear velocity estimation in shaly sandstones from porosity ( ) and clay content ( ) as Also well log studies (Pickett, 1963; Nations, 1974; Kithas, 1976; Miller and Stewart, 1990) indicate a correlation between Vp/Vs values and lithology. Beyond lithology identification, elastic behavior of the material can be known. As a matter of fact, production of sand along with oil and gas is a formidable problem in many younger, unconsolidated rocks. The object of estimating formation strength on the basis of elastic constants is to determine whether the formation is strong enough to produce at high flow rates without sand. If the formation cannot sustain high flow rates without sand, it is beneficial to determine the optimum production rate which can be sustained without producing sand. There is considerable evidence that a good correlation exists between the intrinsic strength of the rock and its elastic constants. The sonic or acoustic log measures the travel time of an elastic wave through the formation. This information can also be used to derive the velocity of elastic waves through the formation. 2. Basic Theory The velocity of the compressional wave depends upon the elastic properties of the rock (matrix plus fluid), so the measured slowness varies depending upon the composition and microstructure of the matrix, the type and distribution of the pore fluid and the porosity of the rock. The velocity of a P-wave in a material is directly proportional to the strength of the material and inversely proportional to the density of the material. Hence, the slowness of a P-wave in a material is inversely proportional to the strength of the material and directly