Feasibility Study of In-situ Combustion in a Carbonate Reservoirs

Abstract

Heavy oil recovery studies is now being carried out in Iran since there is remarkable amount of heavy oil bearing formations in this country. KMO oil field is one of the most important oil fields in which the different recovery methods are being investigated. However, due to very high viscosity the in-situ combustion process seems to be feasible. This field contains heavy oil with very low API degree, near eight, in Sarvak and Asmari formations. To investigate the feasibility of this process, in-situ combustion tube tests on KMO rock and oil have been carried out. Through these experiments combustion front temperature was about 500° C. Additionally combustion front velocity and recovery has been measured. The amounts of oxygen, carbon monoxide and carbon dioxide have been measured by analyzing the effluent gas. One of the risks of running combustion process in carbonate formations is the probability of decomposition due to very high temperature. While decomposition occurs in dolomite or lime stone, the rock will change in to a powder like material that will definitely cause plugging. The carried out tests along with TGA/DSC tests done show that there is no risk of decomposition in KMO reservoir rock around the temperature attained in the combustion front. Finally, the results of the experiments were simulated to predict and investigate the effect of fractures contribution to the process. Introduction With a worldwide resource base that may exceed 6 trillion barrels, heavy oil will be a major energy source for the 21st century as the availability of conventional oil declines. Due to this fact, the development of the heavy oil reservoirs in Iran has gained some momentum recently. In general, the rocks bearing important deposits of heavy crude in Iran are limestone and dolomite which range in age from Cretaceous to Eocene. Heavy oil traps are mainly anticline structures, located in the southwest part of Iran (Zagros area). There are several heavy oil reservoirs in Iran, which are being studied for production. One of the greatest reserves is located in south west of Iran. The most important property of this reservoir is the characteristics of the oil bearing formation which is carbonated and fractured. One of the EOR methods which have been considered for this field is the in-situ combustion process. In-situ combustion is simply defined as burning a part of the oil inside the reservoir in order to generate enough heat to produce the rest of the oil. In the normal method, air is injected through an injection well and oil is ignited in the wellbore, so that a combustion front is formed and propagates through the reservoir and pushes the oil towards the injection well(s)1–2. Running in-situ combustion process in carbonate reservoirs might be risky due to the probability of decomposition of the rock and production of carbon dioxide at high temperatures. The effect of fractures could be sometimes very dramatic. Since fractures may lead to oxygen break through and failure of the process3–4. Besides all these concerns economy and instrumentation requirements are other considerations that should come in to account. Usually, long term investigations and studies are conducted before choosing a reservoir for this process. On of the most important parts of these studies is the feasibility study5. Feasibility studies are carried out in order to understand whether the process is possible on the rock and oil of the field. Combustion tube tests and other thermo metric tests like thermogravimetric analyzert (TGA) and diferenial scanning calorimetry (DSC) are usually used to test the feasibility of a process6–8. Several parameters are either measured or calculated after each test. This work was carried out to determine whether this process is feasible in this reservoir or not. As mentioned before, the first aim was to investigate the risk of decomposition due to high temperature. To reach this point combustion tube tests were done using KMO oil and rock. Forward, reverse, and auto ignition tests have been conducted for this purpose. The forward test was followed by numerical simulation to model the tube and then exerting some fractures in the tube to see whether the combustion initiates or not. Properties of the reservoir have been summarized in table 1.

Experimental and Simulation Study of <i>In-situ</i> Combustion Process in Carbonate Fractured Porous Media

“…This is the zone that has been subjected to the highest temperatures for the longest period and exhibits mineral alteration upon detailed analysis 7) . Running in-situ combustion process in carbonate reservoirs might be risky due to the probability of decomposition of the rock and production of carbon dioxide at high temperature 8) . So majority of laboratory and field researches are done on sandstone formations and research on carbonate formations seems to be less.…”

“…The use of wet combustion began at the moment in which it was recognized that much of the heat stored in the rock during the dry, forward combustion behind the combustion front was being lost to the adjacent formations 8) .…”

Hydraulic Fracturing and Enhanced Recovery in Shale Reservoirs: Theoretical Analysis to Engineering Applications

“…Higher crack spacing reduces the possibility for oxygen to permeate into the matrix, which decreases the temperature and reduces oil recover. 257 A fully coupled simulation model of a fractured well was created to accurately represent the actual gas transport in shale gas reservoirs and was used to assess the effectiveness of thermal recovery on the basis of hydraulic fracture heating. Wang et al 258 created a fully coupled model to study the impacts of fracture heating on the shale gas desorption phenomenon on well performance and production.…”

“…According to the study of Thomas et al, the average temperature of a fracture is higher than that of a shale block without fractures in the early stage, which indicates that more crude oil is consumed during the burning process inside the fracture region than that in the matrix. Higher crack spacing reduces the possibility for oxygen to permeate into the matrix, which decreases the temperature and reduces oil recover . A fully coupled simulation model of a fractured well was created to accurately represent the actual gas transport in shale gas reservoirs and was used to assess the effectiveness of thermal recovery on the basis of hydraulic fracture heating.…”