مجله ژئومکانیک نفت
سال یکم شماره 1 (بهار و تابستان 1396)

History matching is still one of the main challenging parts of reservoir study especially in giant brown oil fields with lots of wells. It would be a challenge in reservoir engineering that due to various parameters and uncertainties in study of reservoirs, many simulation runs are needed to reach a good match for responses in conventional mechanism of history matching. However, for accelerating history matching part, new methods, which are called as assisted or automated history matching (AHM), have been established. In this paper, the latest approach for automated history matching (AHM) has been applied in a real brown field containing 14 wells with multiple responses that is located in south of Iran. Least square support vector machine (LSSVM) has been applied to create proxy model based on cubic centered face method. The optimization algorithms, used in this research, consist of genetic algorithm (GA) and particle swarm optimization (PSO).  Introduction In the latest studies in geosciences and reservoir characterization, employing a proxy model that acts faster, instead of real reservoir model, has led to good results. One of the most important sections in fulfilled study (FFS) and master development plan is history matching, which plays an important role in production scenarios and future production plan of the under study reservoir. In this paper, one of the newest methods is used for making proxy model and then, the model for history matching is optimized.  Methodology and Approaches Least square support vector machine (LSSVM) has been employed to create proxy model based on cubic centered face (CCF) method. The optimization algorithms of genetic algorithm (GA) and particle swarm optimization (PSO) have been used in this research.  Results and Conclusions A new proxy model has been successfully constructed using 1086 samples leading into determination coefficient (R2)

Summary Due to the importance of the rate of penetration and hole cleaning in directional wells to reduce the costs, after detection of critical inclination, the relationship between the effective parameters has been studied in an experimental flow loop. The cuttings, used in the tests, have had a specific size distribution in three types of sand, silica and limestone. The results illustrate that the hole cleaning index (HCI) of powder types in comparison to non-powder types is higher. The effects of bit and pipe rotation on the HCI have been studied in this research work. In this regard, there is an optimum point of the HCI for each of these effects. The end point and the length of the cutting bed have the same behavior. The larger density and lower size cuttings, compared to the lower density and larger size cuttings have higher HCI. In addition, the effect of the shape and size distribution of cuttings on the HCI seems to be remarkable.  Introduction Reparability, durability, cost, drill ability and many other parameters are important in using the bits. Based on these parameters and the softness or hardness of the formation, a bit is selected. The cuttings that are generated in directional drilling, based on the bit type and formation type, have different sizes, shapes, densities and volumes. In this paper, the hydraulic effect after releasing the wait on bit is discussed. Hole cleaning is a key parameter in extended reach drilling, especially in critical inclination periods, where the probability of the different sticking types is increased. The critical inclination is the angle that has the worst hole cleaning. In this research, by applying discrete angles in laboratory tests, one specific angle is determined as the critical point, and in this regard, the effect of size, shape, density and volume of cuttings are studied.

Summary In this paper, in order to minimize drilling costs, it is attempted to offer a way to reduce future drilling risks through analyzing past drilling. Risk management steps are implemented for the past drillings in one of the Iranian oil fields. Before risks can be managed, they must be identified, and they must be identified before they become problems adversely affecting the project. The analysis is the conversion of risk data into risk decision-making information. The most common risks are identified and evaluated in severity and probability.  Evaluated risks are classified into five categories of tolerable (T), low (L), medium (M), high (H) and intolerable (IN). A risk number is assigned to each of these categories that are prioritized based on their risk number. Finally, a consolidated list of priorities of risks is created. Planning turns risk information into decisions and actions for both the present and future. As a new method for risk aversion planning, a three-dimensional (3D) map of the geology formation in order to specify the best future well path, is designed. This shows the high risk and low risk point in the formations that drilling bit will be passed from. Drilling can be carried out in a way that there are tolerable risks.Drilling speed for the formations has been defined as a rock property (same porosity or permeability) and propagated into formations based on the past drilling data with simple kriging method. This study attempts to through an analyzing past drilling from two aspects of time and cost in one of Iranian oil fields as a case study, and then, by implementing risk managements to offer a way to reduce the risk of future drilling in the oil field.   Introduction Drilling sector is one of the most important and most challenging parts of both offshore and onshore oil fields development. A great part of expenses in this development is allocated to the drilling sector. Reduction or minimization of drilling costs is an important step in order to minimize development costs. Drilling costs have become extensively  large in difficult reservoirs with complex drillings so that several companies usually will make groups to share the financial risk (Bourgoyne et. al., 1986). The drilling time that is wasted as the losses such as the loss of drilling equipment and fluids, and the loss of drilling process continuity, is considered as non-productive time (NPT) (Aldred et. al., 1999). Drilling risk management can reduce and even remove these risks and their effects.

 Since this equation does not properly cover geometrical properties of the porous medium of tock, fractal dimensions such as tortuosity and pore space dimensions are used for improving this equation. The aim of this study is to consider and improvement of fractal permeability trends. Therefore, two 3D digital sample namely Bentheimer sandstone and Estaillades carbonate with different pore space complexity and fractal behavior were selected.corresponding porosity and permeability were calculated. Results showed that Kozeny-Carman equation is not a good trend to consider all permeability data. Therefore, fractal equation of permeability was used and their constant values were obtained by a fitting algorithm. Results showed that although fractal equations can consider permeability in a specific sample, they are not applicable for permeability trend evaluation. This is because fractal constants are variable with porosity. To improve these equations, functionality of fractal constant with porosity were obtained and, then, a new equation derived from fractal equation was proposed. This equation was fitted to permeability data and its constant values for each original sample were also obtained. Results showed that this equation can predict permeability-porosity trends better than Kozeny-Carman trend. Also their differences obviously reflect complexity of porous medium of rock.

Summary Fracture toughness is defined as the material resistance against crack extension. Different tests have been presented for determination of mode I fracture toughness of rocks, but results of these tests have considerable differences with each other. In this paper, amongst various methods for determining mode I fracture toughness of rocks, Chevron Bend, Brazilian and Flattened Brazilian tests were chosen for investigation of main causes of difference of fracture toughness values. For all tests, 3-dimensional finite element analysis also was used to find the best test for determining this parameter. Finally, it was concluded that amongst three mentioned tests, the Chevron Bend test is the best for determination of mode I fracture toughness because of the least fracture process zone around the crack tip, fairly good sample preparation, a stable crack extension and the least influence of boundary conditions on fracture toughness values. Introduction Fracture toughness represents the material resistance against crack extension and is one of the most important parameters in hydraulic fracturing test. Because this parameter is one of inherit characteristics of rocks, values of fracture toughness of different tests seems to have a good agreement with each other, but in practice it doesn’t happen. In this paper amongst various methods of mode I fracture toughness determination, Chevron Bend, Brazilian and Flattened Brazilian tests have been selected in order to find main reasons for difference of fracture toughness values. Methodology and Approaches tests were evaluated based on sample preparation, crack extension, crack type, effect of boundary condition and volume of fracture process zone around crack tip according to experimental results and numerical modeling. Results and Conclusions Results showed that Brazilian test is not suitable test because of assuming infinite plate in its equations, crack initiation near loading plates, non-uniform loading condition and effect of boundary conditions on fracture toughness values. In Flattened Brazilian test crack is initiated at the center of sample, fracture toughness is numerically determined based on finite plate assumption and loading conditions is uniform, but fracture toughness values is also affected by boundary conditions. In general, Chevron Bend test is chosen as the best test for determining fracture toughness of rocks according to stable crack growth, which makes natural crack at the point of calculating the fracture toughness, and lower size of the fracture process zone around the crack tip. Also, boundary conditions have no considerable influence on fracture toughness values in this test.

In the oil industry, production of sand particles or detached sand clumps together with the formation fluids is called sand production. Sand production in oil wells is usually related to two fundamental mechanisms. The first mechanism is mechanical instability and degradation of the rock in the vicinity of the wellbore and the other is hydrodynamical instability due to flow induced drag force on the degraded material. In this paper, considering both the mechanisms, a new numerical model for predicting the onset of sanding as well as the amount of sanding is proposed. Implementation of the model in an explicitly coupled flow and deformation finite element program is described. The proposed model by removing the elements that have satisfied the sanding criteria as well as isolated rock chunks from reservoir can capture cavity evolution associated with sand production. The model is calibrated and validated against published results of a sanding experiment in a perforated reservoir rock. Results of the model in terms of initiation and amount of sanding are compares well with the experimental observations which suggests it can be used for sanding analysis of oil wells.