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The significance of unconventional gas reservoirs has been increasing for recent years owing to economic viability of their development, therefore assessment of the challenges and common pitfalls regarding those resources have been gaining importance at the same time. In this regard, the optimization of production performance of these reservoirs with the different well trajectories and completion techniques and identifying the best case scenario become more significant. That is absolutely challenging process due to the several reasons such as ultra-low permeability, desorption effect, and complex geological characteristics. However, it is possible to analyze the various parameters and observe their impact on each system with the help of advances in algorithms, computer power, and integrated software. The objective of this work is to investigate and understand the effect of some reservoir and completion parameters on the future production performance of shale gas and coal bed methane (CBM) reservoirs. A practical model is constructed with the field and synthetic data for the analysis of gas production rate and cumulative gas production versus time in multi-layered shale gas and CBM reservoirs respectively. Changes in the thickness of various stratified layers, permeability, wellbore position, number of hydraulic fracture stage, and also production profile of each system are studied using different well trajectories. The results are obtained by running a series of reservoir simulation conducted by a commercial numerical simulator with dual porosity model for CBM and shale gas reservoirs.
Mud solid and filtrate invasion are the most common causes of formation damage during overbalanced drilling (OBD), which is a method of drilling in which the wellbore pressure remains greater than formation pressure. During the past ten years, underbalanced drilling operations (UBD), which can be defined as a drilling method in which the wellbore pressure is less than formation pressure, has been growing due to their applications such as prevention of lost circulation, reduction of filtration lost in naturally fractured and mature reservoirs.
It is very difficult to always maintain the underbalanced conditions in drilling and completion operations during bit changing and tripping operations. In these cases due to the non-existence of internal and external mud cake, there will be high amount of mud solid and filtrate invasion into the formation.
Besides these, the potential of formation damage still exist during UBD due to Counter-current Spontaneous Imbibition (COUCSI) process. In this case, for water-based muds the original water phase pressure in the reservoir is less than the irreducible water saturation. Therefore the capillary pressure and wettability characteristics of the formation can result in water invasion from the drilling fluid into the formation.
The main goal of this study is modeling of this process for a vertical and horizontal well in a conventional reservoir, which are being derilled using a water-based mud system in an underbalanced mode, investigating the effects of some factors such as amount of underbalanced pressure difference and exposure time. In addition, the amount of invaded water and invasion distance are discussed too. The numerical modeling and resulting governing system of partial differential equations, the descretized forms and initial and boundary conditions are presented.
Based on this numerical modeling, it is shown that for high capillary pressure values, the amount of water invasion increases. Moreover, for lower underbalanced differential pressure and higher exposure times, water invasion and invasion distance increases too.
Formation Damage during Underbalanced Drilling Operation.
In general, during a UBD operation, the effective downhole pressure, which is a combination of hydrostatic mud pressure and physical pump pressure required for fluid circulation, should be less than the prevailing formation fluid pressure of the target formation. UBD conditions may be achieved by the injection of nitrogen, air or natural gas into the circulating drilling fluid. In some cases in which there is a high formation pressure, using a low density drilling fluid can generate underbalanced conditions too. All of these situations permit the reservoir fluid flow into the wellbore while drilling, which is known as flow drilling.
UBD method is being used widely due to its advantages and applications such as rapid indication of producible zones, producing hydrocarbon and conducting well tests while drilling (WTWD), improving production rate and ultimate recovery, increasing rate of penetration, minimizing differential sticking and minimizing formation damage (Oyeneyin 2003; Bennion and Thomas 1994; Qutob 2004; Biswas and Suryanarayana 2004).