This paper describes an experimental investigation of formation damage in a fractured carbonate core sample under underbalanced drilling (UBD) conditions. A major portion of this study has concentrated on problems which are often associated with UBD and the development of a detailed protocol for proper design and execution of an UBD program. Formation damage effects, which may occur even if the underbalanced pressure condition is maintained 100% of the time during drilling operation, have been studied. One major concern for formation damage during UBD operations is the loss of the underbalanced pressure condition. Hence, it becomes vital to evaluate the sensitivity of the formation to the effect of an overbalanced pulse situation. The paper investigates the effect of short pulse overbalance pressure during underbalanced conditions in a fractured chalk core sample. Special core tests using a specially designed core holder are conducted on the subject reservoir core. Both overbalance and underbalanced tests were conducted with four UBD drilling fluids. Core testing includes measurements of the initial permeability and return permeability under two different pressure conditions (underbalanced and overbalanced). Then the procedure is followed by applying a differential pressure on the core samples to mimic the drawdown effect to determine the return permeability capacity. In both UBD and short pulse OBP four mud formulations are used which are: lab oil, brine (3% KCL), water-based mud (bentonite with XC polymer) and fresh water. The return permeability measurements show that a lab oil system performed fairly well during UBD and short OB conditions. The results indicate that a short overbalance pressure provides a OPEN ACCESSEnergies 2011, 4 1729 significant reduction in permeability of the fractured formations. In most tests, even application of a high drawdown pressure during production cannot restore the initial permeability by more than 40%.

This paper describes an analytical approach to investigate the nature of short overbalanced conditions and time effects during underbalanced drilling (UBD) in a naturally fractured reservoir. This study uses an analytical model which is developed for kinetic invasion of mud into the fractures. The model is based on fluid flow between two parallel plates, which is further extended to model the fluid flow in a fractured formation. The effect of short overbalanced pressure and the time effect during UBD as well as the aspects of well productivity and flow efficiency are explained. This model is an Excel-based program and provides a fast and convenient tool for analysis and evaluation of drilling conditions (mud properties, time, and pressure of drilling) in a fractured formation. The model can also predict the impact of the fracture and mud properties on the depth of invasion in the fractured formations.

This paper describes the experimental study of formation damage in naturally fractured reservoir during conventional drilling operations. Dynamic formation damage (DFD) apparatus of NTNU is designed to simulate the drilling fluid circulation process at the formation face in the well bore face under bottom hole condtions. Chalk core samples are selected to study as a representative of very fine-grained limestone formations. A uniform fracture was made entirely along the conventional core. A series of overbalanced leak-off test were conducted with different drilling fluids under different reservoir conditions on the samples. Dynamic leak-off test was performed on the core samples for 4 hours and then it was followed by static leak-off test for 16 hours. Cummulative invasion of mud into the core during leak-off test is measured and then return permeability test and drawdown pressure effects are employed. The experiment results clearly show that drilling moods can cause large irreversible damage to fractured formation and dramatically reduce the productivity of wells producing from natural fracture network. The results present the significant impact of overbalanced pressure on invasion profile. The use of bridging agent such as CaCO3 and polymer play key role in reduction of solid and filtrate invasion in fractured core sample. Return permeability test indicates that cleanup and remove all solids and particles during production is not completed as well. Therfore the experiments recommend the use of drawdown pressure effects to clean-up of solids and particles during production. Introduction The presence of high permeability features in a formation, such as large naturally occurring fractures or extensive interconnected vugular porosity system; represent a significant challenge for overbalanced drilling operations with respect to rapid and deep invasion and significant, often permanent permeability impairment. In most situations, the high permeability of theses fractures and vug systems act as conduits to feed gas or oil from a tight producing source matrix to the wellbore for production. This being the case, the preservation of the high permeability fractures and vugs is of prime importance. The best way to evaluate damage potential is to test representative field fluids and core samples under simulated downhole conditions, as is possible with dynamic formation damage (DFD) test apparatus. Unfortunately, there is very little work reported in the literature investigating formation damage in fractured reservoirs. Jiao et al[1]. described the use of two different bridging agent CaCO3 and acid soluble fibers to reduce solid and filtration into Berea fractured core sample. Their result is recommend the use of fibrous additives are much more effective than granular additives such as CaCO3. Ali et al[2]. reported successful field application of a mixture of different sizes of fiber particles to prevent lost circulation is severely depleted unconsolidated reservoirs. Leopakke et al[3]. studied single- and two particle bridging at a fracture face. They found that if the particle size is not compatible with the fracture width, a stable bridge cannot be performed and a tailored particle size distribution has the best plugging capabilities. Their experimental results show that a mixture of granular partiucles provides the best plug at fracture entrances. The main objective of this study is the experimental study of fluid nvasion in carbonates fractured reservoirs during overbalanced drilling. For this purpose, chalk samples are selected as a representative of very fine-grained limestone formations to simulate in core flood testing. Mud Invasion behavior under different conditions is discussed which infuences by many key parameters such as overbalanced pressure and bridging additives and mud composition (XC-polymer), fracture size, and pore size distribution of carbonate rock.

In order to accurately predict and assess the impact of formation damage on well production or injection models of near wellbore are frequently employed. These models can range in detail and accuracy from basic analytical models to comprehensive numerical models. But which is most appropriate for a specific reservoir or well type? This paper presents a review of the principle options available and their potential applications. In addition a vision of potential future developments in modelling is presented. A constant challenge to the formation damage community is predicting the impact of damage and the value in its reduction or elimination. Thus modelling the impact of damage is usually undertaken in order to answer the question "what does this mean for my well?". Many different model types have been employed over the years. The different approaches, their logic and potential applications in different well and reservoir types are presented together with a brief history of near wellbore modelling. With continuing dramatic advances in computational power and in software, the future of reservoir, near wellbore and well modelling has previously unimagined potential. Some of this potential is also explored and its' applicability discussed. The paper provides a comprehensive overview of near wellbore and formation damage modelling and should provide a useful first entry in to this subject for novices and will no doubt draw constructive responses from the various near wellbore model advocates.

This paper describes an experimental investigation of formation damage during underbalanced drilling (UBD) conditions in a fractured carbonate core sample. A major portion of this study has concentrated on problems, which are often associated with UBD and the development of a detailed protocol for proper design and execution of an UBD program. Formation damage effects have been studied, which may occur even if the underbalanced pressure condition is maintained 100% of the time during drilling operation. One major concern to formation damage during UBD operations is the loss of the underbalanced pressure condition. Hence, it becomes vital to evaluate the sensitivity of the formation to the effect of an overbalanced pulse situation. The paper investigates the effect of short pulse overbalance pressure during underbalanced conditions in a fractured chalk core sample. Special core tests using a specially designed core holder are conducted on the subject reservoir core. Both overbalance and underbalanced tests are conducted with four UBD drilling fluids. Core testing is including the initial permeability measurement and return permeability after two different conditions of pressure (underbalanced and overbalance). Then the procedure is followed by applying a differential pressure on the core samples to mimic the drawdown effect to present capacity of the return permeability. In both UBD and short pulse OBP four mud formulation are used which are, lab oil, Brine (3% KCL), Based mud (Bentointe with XC polymer) and fresh water. The return permeability measurements show that a lab oil system performed fairly well during UBD and short OB conditions. The results indicate that short overbalance pressure provides significant reduction in permeability of the fractured carbonate formations. In most tests, even application of a high drawdown pressure during production cannot restore the initial permeability more than 40%. Introduction The presence of high permeability features in a formation, such as large naturally occurring fractures or an extensive interconnected vugular porosity system; represent a significant challenge for overbalanced drilling operations with respect to rapid and deep invasion and significant, often permanent permeability impairment. In most situations, the high permeability of the fractures and vug systems act as conduits to feed gas or oil from a tight producing source matrix to the wellbore for production. This being the case, the preservation of the high permeability fractures and vugs is of prime importance. It is often very difficult to assess the in-situ-size distribution of fractures (although mico-resistivity logging tools can provide some indications) and many reservoirs may exhibit a wide range of potential fracture apertures, making the design of an effective overbalanced bridging system in such a situation difficult. Theses types of reservoirs may be considered prime potential candidates for UBD operations. However, UBD is a complex process and many factors must be considered and evaluated for candidate reservoirs before any operation. Underbalanced technology may be very successful in reducing or eliminating formation damage if properly executed, but a major portion of this study has centered on problems, which are often associated with UBD and the development of detailed protocol for proper design and execution of UBD program 1. Two main goals are evaluated this study, improving the productivity of fractured reservoir by using UBD and reducing formation damage during UBD. Possible formation damage effects may occur even if an underbalanced pressure condition is maintained 100% of the time during drilling operation. Another, one of the major areas of sensitivity to formation damage during UBD operations is the loss of the underbalanced pressure condition. Hence, it becomes vital to evaluate the sensitivity of the formation to the effect of an overbalanced pulse situation. The best way to evaluate damage potential is to test representative field fluids and core samples under simulated downhole conditions, as is possible with dynamic formation damage (DFD) test apparatus.