Abstract

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.

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