مجله ژئومکانیک نفت
سال دوم شماره 1 (بهار و تابستان 1397)

This paper investigates the crude oil flow through open and rough-walled fractures. The aim of this paper is to evaluate the effects of surface roughness and spatial arrangement of aperture segment on fluid flow phenomenon through rough-walled fractures. In addition, the validity of classic geometrical equations of cubic law correction was explored by comparing the output results of numerical simulations (in terms of permeability). To reach this goal, the crude oil flow through three-dimensional rough-walled fractures was numerically simulated by simultaneous solving of Navier-Stokes and mass conservation equations and utilizing FLUENTTM computational software. The numerical crude oil flow simulation was performed for six different three-dimensional geometrical models of fractures whit different roughness and aperture arrangements and constant porosity. The results of fluid flow simulation were analyzed with different points of view and then compared with the classic geometrical equations of cubic law correction. The results of this study show that (i) for the open fractures, the effects of roughness on the pressure lose is higher than spatial arrangement of aperture segment, (ii) due to the nonlinearity of flow, the permeability of fractures decreases by increasing the Reynolds number, and (iii) with the classic geometrical equations of cubic law correction encounter with about 5 to 35% error and the accuracy of these equations will be decrease by decrease the permeability of fractures or increase of roughness and spatial variability of aperture. These findings prove useful in proper understanding of crude oil flow in fractures, or inclusions in computational simulation of large-scale flow in fractured petroleum reservoirs.

During drilling operation, a volume of formation rocks which have been effected by far field stress is removed. Thus induced stress field redistributes around the borehole wall. While this induced stress field exceeds the resistance of the rocks around borehole wall, borehole breakout will take place. As a result a plastic zone around borehole wall can be observed. Hence, accurate understanding of state of plastic zone around the borehole wall can lead to figure out the behavior of the borehole circumferential environment. The analysis plastic zone around the borehole wall is valuable to determine safe mud window and optimize well trajectory to mitigate well instability problems. On the other hand, analysis the wellbore stability based on the critical pressure may be unreasonable, because the critical pressure can only reflect the degree of difficulty for the initiation of damage at the wellbore rather than the extent of the wellbore damage. Therefore, the results of wellbore stability analysis based on the analysis of plastic zone around the borehole wall are more realistic. In this study, using an elasto-plastic analysis incorporating three commonly used failure criteria as Mohr-Coulomb, Mogi-Coulomb and modified Lade, the extent of plastic zone around a vertical oil well and the condition of stability with specific mud weight in southwest of Iran was investigated. Results showed that according to the Mogi-Coulomb and modified Lade failure criteria which have been confirmed by FMI data the probability of occurrence of plastic zone around the borehole wall was negligible and borehole was in the stable condition. On the other hand, the discrepancy between the results of Mohr-Coulomb criterion and the set of modified Lade and Mogi-Coulomb criteria can be due to the negligence of the effect of averaged main stress.

Today is one of the most important methods of producing hydrocarbon reservoirs, Proper design of wells and casing pipes to have a safe drill operation in sensitive parts of the formations, which itself helps in the production of faster and more long-term storage from the reservoir. Occupation of some oil reservoirs, including the Bangestan reservoir, in deeper parts of the earth, the cause is which will have a more precise design in implementing the wells of this reservoir in different fields. In the field studied, First, we tried to analyze the image log data and the drill information includes the data mud loss and mud weight, fracture of the field is comprehensively studied in addition to a better and more accurate understanding of the expansion of fractures in the reservoir, investigate their relationship with drilling problems and possibility to change the design of the welded pipes and their location more accurately in Bangestan wells, it should be investigated. History of drilling these wells and maximum mud weight required to drill each hole and Condition of installed casing pipes in different wells were studied and wells that have problems and In them, a different design of the casing pipes is used, separated. In this research, the relationship between continuous problems in wells with formations with the highest percentage of problems, was presented and the type of mud with the weight used for drilling problematic formations has been investigated. Finally, by determining the distribution of wells with difficulty In different areas of the field, the density of these wells in each specific area and are areas susceptible to the problem which leads to a change in the design of the casing pipes, identified and introduced.

In naturally fractured reservoirs, hydrocarbon production is mainly depends on permeable fractures. Therefore detecting permeable fractures in reservoir rock is a priority. Generally, there are many methods to detect fractures in reservoirs includes using imaging log, temperature log, mud loss log. Imaging logs are unable to predict if a fracture is permeable or not, therefore It is necessary to use some complementary methods like acoustic methods to detect permeable fractures. In this paper, we analyzed a DSI (Dipole Shear Imager) tool which was taken from one of the hydrocarbon reservoirs in south of Iran. First, by using a commercial software (Geolog), the stoneley wave was separated into direct and reflected parts. Then the reflection coefficient of the stoneley wave calculated in the studied section. It is possible to determine if a fracture from FMI log is permeable or not By comparing it to its reflection coefficient. the Results of this method is promising.

High permeable with low pore pressure formations are one of the main causes in mud cake thickness. This effect influence on differential sticking probability. The fluid loss additives are used widely in drilling operations. In this paper tried to study the effect of these additives on fluid loss on two main categories of drilling fluids, salt saturated and lignosulfonate muds, with help of mud cake characterization equipment and API filtration setup. Also there are some reference muds for result comparison and tool calibrations. The results indicate that salt saturated muds have more fluid loss and filtration problems than lignosulfonate and reference muds. Also the shape of the additives has the main effect on these effects. In salt saturated muds wallnut shell was the best fluid loss reducer and cellephone had the most probable effect on differential sticking. In lignosulfonate muds mica had the best fluid loss reducing agent and sawdust Had the worst effect on sticking.

Porosity as the dominant inherent property of rock has important applications in rock mechanics particularly porous media of rock such as oil reservoirs.  The heterogeneous properties of rock are related to the inherent characteristics which control the behavior of rock under stresses. If one factor such as porosity varies other inherent variables of rock are affected and effect of porosity on the mechanical behavior and properties of rock cannot be clear.  To find homogenous rock specimens having different porosities with the same others inherent properties such as gains sizes, is a difficult task or it seems impossible.  Therefore, physical modelling can be a useful tool to make a rock like material having different porosities as the other inherent factors can be kept constant.  In this paper, the constituents of cement, water and filler material having a particular granularity in two types using sand and silica have been used to make the model material.  Two techniques including compacting the mixture of model material and changing the water content of model material, have been applied to change the porosity and cylindrical specimens have been made.  Porosity and unit weight of specimens have been measure with accuracy of 0.01.  A homogenous rock like material having a wide ranges of porosities from 9.00% to 34.09% have been developed that is similar to natural sandstone.  Uniaxial compressive strength has low scatter with varying the porosity due to the homogeneity in comparison with natural sandstone.

Wave propagation across the rock masses is one of the most topics in rock dynamics, which is used in various petroleum, mining, civil military industries. Since rock mass includes rock material and various forms of discontinuities, the nature of rock mass discontinuities significantly affects mechanical properties and engineering behavior. Therefore, adequate knowledge and understanding of how the wave propagates in rock mass, especially for projects that are more important and more sensitive, is necessary. In the present paper, the analytical method for wave propagation across rock mass based on the method developed by Huang et al. (2014) is presented for elastic state and wave propagation in single-joint and multiple parallel joints is studied. Then, a parametric study was performed on parameters including normalized joint stiffness, joint dip, normalized spacing and number of joints on wave propagation in single joint and multiple parallel joints and the obtained results have been compared with the results of other proposed methods.