مجله ژئومکانیک نفت
سال سوم شماره 3 (پاییز 1398)

Determination of the rate of penetration is one of the most important factors in oil industries. Generally, two methods have been proposed for modeling the rate of penetration which include physical and artificial neural networks methods. Artificial neural networks can be used accurately in order to predict the rate of penetration in which the prediction of the rate of penetration does not include experimental coefficients and bit specifications. Furthermore, in this method, the rate of penetration only depends on the input data. In this paper, the rate of penetration using almost 2000 daily drilling reports and rock mechanical properties was applied. These data were reduced to approximately 1800 data according to data preprocessing. The rate of penetration was modeled by two artificial neural networks including a Multi-Layer Perceptron and an Elman with a hidden layer. After preprocessing the input data and sensitivity analysis of the number of neurons in the hidden layer, seven neurons were chosen as the optimized number of neurons in the Multi-Layer Perceptron with the correlation and mean square error of 77.1% and 1.31, respectively. Also, the Elman neural network showed the correlation and mean square error of 77.6% and 1.33, respectively. Thereafter, the fuzzy AHP method was applied for imposing weights, gaining by expert comments, on the input data resulted in improvements of the artificial neural networks. The results of this investigation have shown insignificant superiority of the Multi-Layer Perceptron neural network for prediction of the rate of penetration comparing to the Elman neural networks. Therefore, the proposed weighted Multi-Layer Perceptron neural network models the rate of penetration accurately and appropriately using available data.

Hydraulic fracturing is one of the most important stimulation methods for oil and gas reservoirs to increase fluid flow from low permeability reservoirs to wellbores. Various factors, such as in-situ stresses, joints and natural fractures of the formation, fluid rheology, mechanical properties of the formation, injection fluid flow rate and perforation operation, effect on the pressure and hydraulic fracture geometry. In this research, for the experimental investigation of the hydraulic fracturing, considering the reservoir condition, a three-axial machine with the ability to apply the main stresses was designed and built. Then, 32 concrete cubic samples with 10 × 10 × 10 cm dimensions were constructed and cured in the laboratory and the effect of the in-situ stress field and stress regime on the geometry and breakdown pressure, the pressure-time diagram, the pattern of crack propagation and finally the cross fractures in both vertical and horizontal wellbores were investigated. The results showed that increasing the maximum horizontal stress in the vertical wellbore leads to increased breakdown pressure and increasing deviatoric stress in the horizontal wellbore reduces the breakdown pressure.

Wave propagation across rock masses is one of the most important topics in rock dynamics, which is used in various petroleum, mining, civil and military industries. Since rock masses contain rock material and various forms of discontinuities, the nature of rock mass discontinuities significantly affects on the mechanical properties and engineering behavior of rock messes. Therefore, there is a need for adequate knowledge and understanding of how the wave propagates across into the rock masses, especially for projects of higher importance and sensitivity. The purpose of this paper is numerical modeling of wave propagation in single-joint and parallel joints of rock masses. In this regard, the behavior of rock deformation is assumed as linear. In the present study, the numerical study of wave propagation and the effect of normalized joint stiffness parameters and the angle of incidence in single-joint rock masses on wave propagation is done using UDEC software. Also, the propagation of the wave through the rock mass with several parallel joints has been studied and the effect of the normalized spacing between the joints and the number of joints on the wave propagation has been investigated and, finally, compared with the results of the analytical methods of the researchers.

Borehole instability in fractured formations and fluid loss during drilling is an important challenge for creating a stable drilling pattern in the oil industry. Understanding complex mechanisms and coupled hydromechanical phenomena which occur in fractured formation, needs to be more studies by using powerful approach such as distinct element methods. One of the main causes of fluid loss and increased in drilling costs, is the presence of fractures in the rocks. because of that, the investigation of borehole instability in fractured formation would be useful in determination of a safe methodology for drilling processes. In this paper, the simulation of the hydromechanical conditions of a borehole in fracture formation is carried out using Discrete Fracture Network (DFN) and Distinct Element Method (DEM). The modeling presented in this paper is based on real geomechanical and fractures characteristics in one of the boreholes in Persian Gulf oilfield. The model is validated using normalized yield zone criteria according to the caliper log data. The investigation of mud loss in the fracture network is based on changes in volume of spaces, between the discontinuities and non-compressible assumption for drilling fluid. In this paper, in other to understanding the effect of fracture density on borehole stability and fluid loss, six different scenarios for fracture density was investigated. The results showed, increasing the fracture density can be an absolute negative factor for instability in borehole. The numerical analysis shows that, in a specific range of fracture density, the overall mechanism of shear deformation in borehole would be different. However, the result of fluid loss analysis in fractured formation adjustment of borehole, showed that, by increasing the fracture density, volume of fluid loss increased continuously.