Serhat Akin

Computed tomography in petroleum engineering research

Abstract Imaging the distribution of porosity, permeability, and fluid phases is important to understanding single and multiphase flow characteristics of porous media. X-ray computed tomography (CT) has emerged as an important and powerful tool for non-destructive imaging because it is relatively easy to apply, can offer fine spatial resolution and is adaptable to many types of experimental procedures and flow conditions. This paper gives an overview of CT technology for imaging multiphase flow in porous media, the principles behind the technology and effective experimental design. By critically reviewing prior work using this important tool, we hope to provide a better understanding of its use and a pathway to improved analysis of CT-derived data. Because of the wide variety of image processing options, they are discussed in some detail.

Spontaneous imbibition characteristics of diatomite

Abstract A systematic investigation of fluid flow characteristics within diatomite (a high porosity, low permeability, siliceous rock) is reported. Using an X-ray computerized tomography (CT) scanner, and a novel, CT-compatible imbibition cell, we study spontaneous cocurrent water imbibition into diatomite samples. Air–water and oil–water systems are used and the initial water saturation is variable. Mercury porosimetry and a scanning electron microscope (SEM) are employed to describe diatomite pore structure and the rock framework. Diatomite exhibits a fine pore structure and significant pore-level roughness relative to sandstone thereby aiding the flow of imbibing water. Despite a marked difference in permeability and porosity as compared to sandstone, we find similar trends in saturation profiles and dimensionless weight gain vs. time functions. Although diatomite is roughly 100 times less permeable than sandstone, capillary forces result in a strong imbibition potential for water such that imbibition rates rival and surpass those for sandstone

Estimation of fracture relative permeabilities from unsteady state corefloods

A methodology to test the possible deviations of fracture relative permeabilities from straight-line relationship with saturation is discussed. It is shown that with matrix relative permeability data known, it is possible to obtain fracture relative permeabilities by history matching experimental data with double porosity numerical models output. Rather than linear saturation dependence, fracture relative permeability is a power law function of the corresponding phase saturation. The impact of several important parameters like double porosity formulation, capillary pressure and shape factor is discussed. It has been found that double porosity formulations (other than the double permeability model) could represent the flow in a fractured medium.

Application of artificial neural networks to optimum bit selection

Optimum bit selection is one of the important issues in drilling engineering. Usually, optimum bit selection is determined by the lowest cost per foot and is a function of bit cost and performance as well as penetration rate. Conventional optimum rock bit selection program involves development of computer programs created from mathematical models along with information from previously drilled wells in the same area. Based on the data gathered on a daily basis for each well drilled, the optimum drilling program may be modified and revised as unexpected problems arose. The approach in this study uses the power of Artificial Neural Networks (ANN) and fractal geostatistics to solve the optimum bit selection problem. In order to achieve this goal a back-propagation ANN model was developed by training the model using real rock bit data for several wells in a carbonate field. The training and fine-tuning of the basic model involved use of both gamma ray and sonic log data. After that the model was tested using various drilling scenarios in different lithologic units. It was observed that the model provided satisfactory results.

A laboratory study of single-well steam-assisted gravity drainage process

Abstract An investigation of the optimization of startup procedure for single-well steam-assisted gravity drainage (SW-SAGD) was made as the project economics are influenced significantly by the early production response. An experimental investigation of two early-time processes namely cyclic steam injection and steam circulation to improve reservoir heating is discussed and compared to continuous steam injection as well as other well configurations like vertical injector–horizontal producer and horizontal injector–horizontal producer. Crushed limestone saturated with heavy oil (12.8° API) and water was packed in a laboratory model for the experiments. The effectiveness of the methods is compared within themselves and to conventional steam-assisted gravity drainage (SAGD) by measuring the size of the steam chamber as a function of time. The steam chamber area for cyclic steam injection is slightly greater than that of steam circulation case. Furthermore, numerical simulation studies of different early-time processes were conducted and compared to the experimental data using a commercial simulator. It was observed that the numerical model results underestimated the cumulative oil recovery and the steam chamber size. Results from this study, including cumulative recoveries, temperature distributions, and production rates display the differences between the methods.

Oxidation of Heavy Oil and Their SARA Fractions: Its Role in Modeling In-Situ Combustion

In situ combustion is a thermal recovery technique where energy is generated by a combustion front that is propagated along the reservoir by air injection. Most of the previously conducted studies report thermal and fluid dynamics aspects of the process. Modeling in situ combustion process requires extensive knowledge of reservoir data as well as reaction kinetics data. Unfortunately, limited kinetic data are available on the rates and the nature of partial oxidation reactions and the high-temperature combustion reactions of crude oils and their saturate, aromatic, resin, and asphaltene (SARA) fractions. Moreover, the impact of such data on the modeling of the in situ combustion process has not been investigated thoroughly. Thus, we modeled in situ combustion experiments conducted on a 3D semi-scaled physical model that represents one fourth of a repeated five spot pattern. In all experiments a vertical injector is employed whereas, both vertical and horizontal producers have been installed to recover two different crude oils (heavy and medium). Several locations for the producers have been tried while keeping the length of the wells constant: vertical injector-vertical producer, vertical injector-horizontal side producer, and vertical injector-horizontal diagonal producer. In these experiments diagonal producers performed better than the others. We first simulated the experiments by incorporating a kinetic model that is based on grouping the products of cracking into six pseudo components as heavy oil, medium oil, light oil, two non-condensable gases and coke using a commercial thermal simulator (CMGs STARS) Four chemical reactions were considered: cracking of heavy oil to light oil and coke, heavy oil burning, light oil burning, and coke burning. Most of the experiments were history matched successfully with the exception of ones where a diagonal horizontal producer was used. We then repeated the simulations using SARA kinetic parameters. We observed that all matches were somewhat improved. We finally present a discussion of application of the models to field scale.

Genetic algorithm for estimating multiphase flow functions from unsteady-state displacement experiments

Abstract Relative permeability and capillary pressure are the primary flow parameters required to model multiphase flow in porous media. Frequently, these properties are estimated on the basis of unsteady state laboratory displacement experiments. Interpretation of the flood process to obtain relative permeability data is performed by one of two means: application of frontal advance theory or direct computer simulation. Application of frontal advance theory requires a number of experimental restrictions such that the pressure drop across the core is sufficiently large that capillary effects, particularly at the outlet end of the core, are negligible. A parameter estimation technique overcomes significant limitations of the classic calculation procedure. In this approach, functional representations or point values are chosen for the relative permeability curves. Adjustable parameters are then picked to minimize a least squares objective function. Previous applications of this approach have used Gauss–Newtons method with or without Marquardts modification. More recently, a simulated annealing method was also utilized. In this study we propose an interpretation method using recently developed genetic algorithms. The advantage and convenience of a genetic algorithm is that the method converges in all situations to a global optimum unlike Gauss–Newton methods, and it is as fast as the simulated annealing method. The performance of the algorithm is demonstrated with data from hypothetical and laboratory coreflood-displacement experiments where a computerized tomography scanner is utilized. It has been determined that the performance of the algorithm depends on the probabilities of crossover and mutation, and the proper usage of the fitness function.

Heavy-oil solution gas drive: a laboratory study

Abstract When some heavy-oil reservoirs are produced using solution gas drive, they show: (1) higher than expected production rates, (2) low produced gas–oil ratio, and (3) relatively high recovery. The reasons for this behavior are not clear. A series of X-ray computerized-tomography (CT)-monitored, heavy-oil pressure depletion experiments were carried out to examine the core-scale phenomena using high pressure/high temperature equipment. Viscous white mineral oil ( μ =220 cp at 20 °C) and 9° API heavy crude oil from the Hamaca region of the Orinoco Belt, Venezuela, were used. A transparent cell attached to the outlet of the sand pack allowed monitoring of bubble size and shape as bubbles exited the sand pack. Conventional solution-gas-drive behavior was observed in the experiments conducted with the mineral oil: large regions of pore space within the core became saturated with a continuous gas phase and ample gas mobilization was witnessed. In the heavy-crude-oil experiment, however, it was inferred that gas bubbles were of slightly greater size than pore dimensions. The fraction of gas mobilized was not large. The difference in behavior between mineral oil and crude oil results suggests an effect due to large oil-phase viscosity, relatively rapid depletion rate, and possibly the high asphaltene content of Hamaca crude oil. Critical gas saturation was gauged as the saturation at which mobile gas was first observed regardless of whether the gas was continuous. For both experiments, the critical gas saturation was observed to be around 3% to 4%.

Field-Scale Analysis of Heavy-Oil Recovery by Electrical Heating

SummaryElectrical heating for heavy-oil recovery is not a new idea, but the commercialization and wider application of this technique require detailed analyses to determine optimal application conditions. In this study, applicability of electrical heating for heavy-oil recovery from two heavy-oil fields in Turkey (Bati Raman and Camurlu) was tested numerically. The physical and chemical properties of the oil samples for the two fields were compiled, and in-situ viscosity reduction during the heating process was measured with and with-out using iron powder. Iron powder addition to oil samples causes a decrease in the polar components (such as carboxylic and phenolic acids) of oil, and the viscosity of oil can be reduced significantly because of the magnetic fields created by iron powders. Three different iron-powder types at three different doses were tested to observe their impact on oil recovery. Experimental observations showed that viscosity reductions were accomplished at 88 and 63% for Bati Raman and Camurlu crude oils, respectively, after 0.5% iron (Fe) addition, which was determined as the optimum type and dose for both crude-oil samples. Next, field-scale recovery was tested numerically using the viscosity values obtained from the laboratory experiments and physical and chemical properties of the oil fields compiled from the literature. The power of the sys-tem, operation period, and the number of heaters were optimized. Economic evaluation performed only on the basis of the electricity cost using the field-scale numerical modeling study showed that the production of 1 bbl petroleum costs approximately USD 5, and at the end of 70 days, 320 bbl of petroleum can be produced. When 0.5% Fe is added, oil production increased to 440 bbl for the same operational time period.IntroductionCrude oils whose API gravity is smaller than 20 are called heavy oil (Conaway 1999). The key to produce oil from these resources is to reduce oil viscosity, and that is best accomplished by heating these resources, which can be achieved by thermal methods (i.e., hot-fluid injection, in-situ combustion, and thermal stimulation) (Farouq Ali 2003; Prats 1982). Apart from the common thermal methods, electromagnetic heating and electrical heating can also be considered as alternative thermal methods. While steam-based methods have been more successful economically and techni-cally than others, alternative heating methods were found to be uneconomical for heavy-oil recovery because of the high operating costs in the past (Thomas 2007). Because of recent increase in oil prices, the electrical heating technique could be considered as a commercial method (Campbell and Laherrere 1998). Electrical-heating tools and their applications can be divided into three categories on the basis of the frequency of electrical current used by the tool (Sahni et al. 2000):(1) Low-frequency currents are used in resistive/ohmic heating.(2) High-frequency currents are used in microwave heating methods.(3) Induction tools have the ability to use a wide range of low- to medium-frequency currents, depending on heat requirements and desired temperature.These methods are applied in the field by using a downhole mag-netron or heater (Prats 1982). Heating with frequencies less than 300 kHz can be described as electrical-resistance heating (ERH) (Maggard and Wattenbarger 1991). This mode of heating for petroleum recovery has been known since the late-1960s. Reservoir-simulation models (Rangel-German et al. 2004; Sierra et al. 2001) and experimental models (Newbold and Perkins 1978; Amba et al. 1964) have been used in the past to study electrical heating. Electromagnetic heating such as microwave heating for recovery of heavy oil from thin pay zones, was studied experimentally (Jha and Chakma 1999; Acar et al. 2007). In order to enhance the electromagnetic heating efficiency, use of receptors such as activated carbon, iron oxides, and polar-ized solvents has been proposed (Jackson 2002). In this study, applicability of electrical heating for heavy-oil recovery from two heavy-oil fields in Turkey (Bati Raman and Camurlu) was tested experimentally and numerically. Experimen-tal studies were conducted to study the efficiency of the method. In addition, to reduce the viscosity of oil, different types of iron powders were used. Experimental results coupled with the data available in the literature were used to simulate the process numeri-cally at the field scale. TheoryAs an electromagnetic wave that is radiated from the electrodes into the oil-bearing formation propagates into the formation, flu-ids and other reservoir materials impede its passage by providing resistance to the flow. As a result, the intensity of the propagating wave is reduced and the energy is converted to heat. There are key differences between low- and high-frequency heating. At low frequencies, resistance heating dominates compared to dielectric heating that dominates at higher frequencies. Heat transfer from an electromagnetic-wave source to a porous medium can be described by the energy equation. Evolution of temperature as a result of electrical energy can then be obtained by the heat equation, with the following modification:

Relation between mechanical stiffness and vibration transmission of fracture callus: An experimental study on rabbit tibia

Abstract It has been suggested that the vibration transmission across a fracture is affected by the stages of healing of the fracture callus. This study aims to correlate the change in vibration transmission with mechanical stiffness of the callus measured by three-point bending. The right tibiae of male, three-month old local albino rabbits were osteotomized and stabilized by intramedullary fixation following open reduction. The intramedullary rods were removed on the 15th, 28th, 42nd and 90th days postoperatively and the tibiae were excised for vibration, three-point bending and bone mineral density analysis by quantitative computerized tomography (QCT). Optimum time for clinical weight bearing was determined by checking the convergence of the vibration parameters of the fractured tibia to those of the unfractured contralateral. The conclusions obtained from curvature analysis, based on vibration experiments, were in considerable correlation (Spearmans rank correlation coefficient r = 0.93, p = 0.003) with the conclusions obtained from the three-point bending test data which reflected the mechanical condition of the bone by direct means. However, no correlation between bone mineral density change and vibration transmission was noted.