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.

Underbalanced drilling (UBD) has several advantages compared to conventional drilling. These advantages include the elimination of formation damage, higher penetration rate, reducing circulation loss and the possibility of actually producing hydrocarbons during the drilling process. UBD technology and applications have recently been applied while drilling challenging wells in Iranian fields. It is generally accepted that the success of underbalanced drilling is dependant on maintaining the wellbore pressure in an operational window determined by the formation pressure, wellbore stability, and the capacity of the surface equipment. There are several models which can predict the wellbore pressure. Traditional models are mostly empirical and lead to acceptable results for specific conditions but fail for other conditions. In the last decade of the last century, some mechanistic models have been developed which result in an acceptable range of outcomes for a wide variety of reservoirs. On the other hand due to the dynamic nature of the process, some researchers have recently focused on development of dynamic models. This paper presents an improved, comprehensive, mechanistic model for pressure prediction through a well during UBD operations. The comprehensive model consists of a set of correlations for predicting flow pattern and estimating the pressure in addition to two-phase flow parameters in bubble, dispersed bubble, and slug flow. On the other hand the most recently developed empirical correlations have been applied to determine PVT properties. The accuracy of these correlations has been tested in more than 20 oil wells in Iran. Naseri et al. and Almarhoun correlations was applied to determine the live and dead oil viscosities. Naseri et al. model which was originally developed for Iranian reservoirs was used in our model and the results are promising.

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Toe-to-heel air injection (THAI), a method of in-situ combustion, was studied in this research. The temperature distribution was analytically formulated and compared to the experimental data. The objective of this study was to investigate the application of horizontal wells to thermal recovery via the gravity drainage mechanism, both analytically and experimentally. To do so, first, the differential equations of heat transfer around the moving burning front in a vertical combustion tube were derived. Solutions of those equations were obtained. Then, the recorded temperatures within the combustion tube were compared to the results of the analytical model. Finally, new "type-curves" were generated for approximating the front velocity factor which seemed to be applicable to process optimization.

A field case analysis of under balanced drilling (UBD) in the "A" structure located in Southern Iran has been carried out in the present paper. It was initially reported that 51% of an average well's cost was Non-Productive Time (NPT). Typical recordable NPT categories and key performance indicators used include tight hole, tool failure, hole cleaning issues, well control and lost circulation. It was decided that aerated drilling could be applied with advantages such as higher penetration rates, less lost circulation and overall lower drilling cost. As part of designing these wells the bottom hole pressure (BHP) was minimized. This paper shows that in the planned UBD, pre-simulated BHP is in good agreement with the operational BHP. The well drilling design of mud and air rates and the corresponding pressures are in this paper have been plotted against the field recorded pressures for different mud and air rates. The result of the pre-simulations also revealed that there is an unfavorable range of mud flow rate that provides a low BHP of the aerated mud for different mud rate and air injection rates. By illustrating the BHP (dynamic and static), annulus back pressure and different mud rates, it has been shown that an optimum combination, of mud and air rates must be determined in order to maximize the penetration rate. The design process of BHP includes checking for required cuttings carrying capacity, which is determined by ensuring that kinematics energy per unit volume is enough for all planned rates. The design method presented herein also suggests injecting air into mud during drilling of the lost circulation intervals as the best mud loss controlling method. The methodology and the calculation procedures used to pre-design the operation are presented herein with the field data against the pre-estimated. The results of this approach in the field have given reduction in NPT with some results presented herein. Introduction A well was drilled with an aerated drilling program to reach the "A" structure located immediately to the east of the central Iranian fault along with Dashtak and Kutah structures, in Fars North area. Close by, on the western side of the fault, other fractured gas and oil fields are located. During drilling of well "A-1", the larger challenge was controlling mud weight to avoiding lost circulation, tight hole and wellbore collapse (Figure 1). At the depth of 3850m, loss of circulation with rate of 90–170 bbl/hr was observed, which was controlled with LCM (Lost Circulation Material) after about 3.72 days. Drilling operation continued with 18–42 bbl/hr losses down to the depth of 3890 m and to 3928 m with 67.5 pcf mud and 8–21 bbl/hr losses. At 4242 m the mud weight was decreased to 65.5 pcf and well started to flow at 20 bbl/hr. At this point the mud weight was gradually increased to 67.5 pcf while continuous loss and salt water flow was occurring. It is believed that the well could only have been drilled with aerated drilling practices because of the specific challenges encountered.