مجله ژئومکانیک نفت
سال پنجم شماره 4 (زمستان 1401)

Oil wells & boreholes logs data are interpreted/processed to identify petrophysical, mechanical, and in-situ geomechanical properties for rocks around oil wells, due to high cost and geological problems, some well logs cannot be measured. For example, sonic logs contain geophysical, and geomechanical information critical to determining the modulus of dynamic elasticity, Young's modulus, the bulk modulus, acoustic resistance/impedance, the shear modulus, and the Poisson's ratio of rocks around the well’s wall. Therefore, in this paper, two random wells were selected from one of the oil fields in southern Iran, one of which was selected as training well to determine the appropriate model and the other to predict the shear and compressional wave slowness. Data analyses were performed using a range of machine learning methods and setting hyperparameter tuning on algorithms, the best models were selected for predicting/estimating sonic logs. In this process, among the regression methods, the K-nearest neighbors algorithm (KNN), and among the combined methods, the Random Forest Regression algorithm and the Extra Tree Regression algorithm show the highest correlation coefficient. As a result, the extra tree algorithm for modeling was performed on the training sets and testing sets data of the well. Then this model was used to predict and synthesize the slowness acoustic compressional and the slowness acoustic shear of the target well. Then, by comparing the actual data of the target well, the root mean square error and the R-squared were obtained. Then, using poroelastic equations, the field stresses were determined and found that Sarvak and Ilam reservoirs are in reverse stress regime and Asmari reservoir is in normal stress regime up to Strike-slip. At the end of this article, using the rock mechanics criteria, the best optimal safe mud weight windows in the studied was presented.

One of the most important aspects of governing physical processes through rough-walled fractures is the non-linear behavior of flow. The onset of non-linear flow in rock fractures is characterized by critical Reynolds number, which is the main object of this paper. In this paper, the technical aspects of non-linear flow of crude oil through open rock fractures and critical Reynolds number were studied. These issues were studied based on the results of three-dimensional numerical simulation of the Navier-Stokes equations for crude oil flow through rough-walled fractures. The finite volume simulation of crude oil flow through three-dimensional space of rough-walled fractures was performed for a wide range of Reynolds numbers or flow rates. The results of crude oil flow simulations were analyzed based on the Forchheimer’s law. The coefficients of energy losses due to viscous and inertial dissipation mechanisms were derived from the Forchheimer’s law regression on the results. Then, the role of linear and nonlinear energy losses with viscous and inertial dissipation mechanisms was evaluated based on the relation between Reynolds and Forchheimer non-dimension numbers. Finally, the critical Reynolds number was determined for onset of non-linear flow through rough-walled fractures. The results of this study indicate that the Forchheimer’s law appropriately describes the non-linear crude oil flow through rough-walled fractures. By increasing the Reynolds number (or flow rate), the ratio of viscous energy lose from total energy losses decreases non-linearly and simultaneously, the inertial dissipation becomes the dominant mechanism of energy lose. The critical Reynolds number for crude oil is in the range of 30 to 46 for the open fractures of this study. The maximum and minimum of critical Reynolds number follow the minimum and maximum of inertial dissipation coefficient and also the gradient of Reynolds- Forchheimer curve, respectively.

Determining the hydrocarbon reservoir zone is one of the challenges for researchers in this field. So far, various approaches have been proposed in the field of hydrocarbon reservoir zoning. In the meantime, the question that arises is whether the potential of different zones of a hydrocarbon reservoir can be separated using geomechanical parameters? For example, in a sandstone reservoir, can the sandy and shale zones be separated? To answer this question, it is necessary to calculate some of the geomechanical parameters of the reservoir. Therefore, in this study, to obtain geomechanical parameters, three-dimensional seismic data of one of the oil fields in southern Iran were used along with the data of four existing wells. In this way, first inversion was performed on three-dimensional pre-stack seismic data, and three parameters, acoustic impedances (S and P waves) and density were extracted. Then, using the results of the inversion step, geomechanical parameters including elastic modulus, Lame parameters (LMR) and brittleness were calculated. In the next step, using the calculated geomechanical parameters, the reservoir zone was well separated from the non-reservoir zone in terms of hydrocarbon potential in time sections. Then, the calculated geomechanical parameters were verified using scatter diagrams and by comparing with the logs of the existing wells. The correlation value for the acoustic impedance was more than 90%.

After picking the Johnson horizon, we continue the processes related to making the window connected to the horizon. The output contains SEGY data, which has the information on the amplitude and frequency of traces in the Johnson horizon window. Figures 4 and 5 are the results of quantitative analysis of seismic facies that show the correlation coefficient (R) in terms of inline and crossline. In each of the shapes, one of the wells is considered as the base and the subsequent trace at the location of that well will be the main trace. Facial (seismic) changes in all shapes show a similar general trend. The number of traces extracted from the Johnson window is 84,2001. Twenty samples are provided for each trace to determine the shape of each trace in the depth of the window. As a result, using amplitude values to increase time, it determines the waveform of each trace for us. Thus it is clear that the waveform attribute, which is a dual attribute of the amplitude and frequency composition, has been extracted for the whole set of tremors. Using the correlation coefficient relationship, the similarity of each of these traces is compared with a baseline trace. The connection between seismic facies around the well and the existing depositional facies is made with more certainty. But it is possible to study the gradual change of sedimentary deposits and facies and compare them with seismic facies changes. By studying the results of wells in the area and geological information, it is possible to show some sedimentary changes along with seismic changes in the Johnson horizon window. The predominant lithology of the northwestern part is limestone or limestone with clay deposits. This is the northwestern part where the Poseidon 2 and Boreas wells are located.

The perforating operation with the shaped charged includes creating a hole in the casing, and the cement around it, and the production formation in order to create a connection between the reservoir and the oil well. Several factors affect the efficiency and depth of penetration of perforation. The role of the real triaxial stress, loading condition and the number of chaped chrages on the efficiency of perorating has not been well investigated. Therefore, a large-scale device for perforate block samples under real three-axis conditions was designed and made. In this study, in order to optimize the number of large-scale tests, the Taguchi test design method was used and the contribution of each of the insitu stresss on the penetration depth was determined using the statistical analysis of the results. The results simulate the conditions of n-situ stresses on vertical and horizontal wells in the fault zones, respectively, strike-slip and reverse. The results showed that the depth of penetration is maximum in surface conditions. The stress parallel to the firing axis has the least effect in reducing the depth of penetration. In the gun peroration, the penetration depth of the first charge is much lower than the second and third ones. The shaped charges made a hole and some tensile cracks around it. The propagation pattern of tensile cracks depends on loading condiion. The number of tensile cracks created around the hole in limestone is much higher than in concrete. The diameter rduction along the first & second hole is different.

Drilling fluid lost circulation into underground formations, in areas of high permeability, formations with natural fractures, and weak formations is one of the common problems in oil and gas well. Some researches show that about 20% of the non-productive times during drilling operations are due to lost circulation. Generally, the remedial and preventive methods are the two main approaches to deal with the problem of lost circulation. One of the methods of preventing drilling fluid lost circulation is to use the stress cage method in order to increase the formation fracture pressure around the well. Several factors including rock mechanical parameters, properties of bridging materials, stress regime, and properties of drilling fluids are significant in the effectiveness of the technology. In this research, the influence of the horizontal stress ratio on the effectiveness of the stress cage technology has been investigated. For this purpose, first, the three-dimensional model of the well is presented, taking into account the elastic behavior of the rock, and then the tangential stress changes in two states before creating a fracture and after bridging the fracture have been investigated. The results showed that, based on the data used, the application of this technology leads to an increase of 3618 psi in the tangential stress at the wellbore wall and, as a result, an increase in the formation's fracture pressure. In weak formations, this prevents induced fractures and drilling fluid lost circulation. Also, the maximum tangential stress was observed at bridging location of 0.5 inches from fracture aperture. On the other hand, the results showed that the stress cage’s effectiveness depends on the horizontal stress ratio. Based on the results of the numerical model, at the maximum horizontal stress to minimum horizontal stress ratio of greater than 0.715, the stress cage technology effectively increases the tangential stress.