Utilizing Real Time Logging While Drilling Resistivity Imaging to Identify Fracture Corridors in a highly fractured Carbonate Reservoir

Abstract

The Minagish structure in southwest corner of Kuwait is a multi reservoir field. One of the potential reservoirs is the Mishrif formation. Developed as a limestone, sedimented in a mid-ramp environment, it generally consists of fine-grained packstones to wackstones that is highly bioturbated. The average thickness is about 300 ft with an average Net of 170ft in the upper layers. An average porosity value will be around 15% and permeability ranges between 0.001-17 mD. The oil in the Mishrif is highly viscous and production is normally enhanced by fractures in the upper Mishrif layers as they act as the main permeability conduit for the main storage below. The second Mishrif layer unit 2 (M9 & M8) is a fairly high porous peloidal packstone to grainstones sequence that is highly fractured at the upper 15 feet of the layer's "dual porosity system". The fracture corridors within the layer improve permeability, thereby making it a good potential for horizontal well placement. It was impossible to reach the observed production rates from matrix without one or two major fluid conductive fracture corridors. The main storage Mishrif layer unit 3 (M7 & M6) is within a 15% porosity layer that is 10–15 ft thick and contains a large volume of oil. Previous wireline image studies carried out identified these layers and their corresponding fractures, but because most of the studied wells were vertical, the fracture corridors could not be properly related to presence of faults in the field. This paper aims at showing how effective Logging While Drilling (LWD) resistivity images based on laterolog principle were useful in real time to identify the different layers within the Mishrif reservoir and establish the relationship between the fracture corridors and the intersected faults in a long horizontal well that posed a risk to data acquisition with standard wireline pipe conveyed logging. Introduction The Minagish (MN) field, in West Kuwait, is a North-South trending anticline (Fig.1) with hydrocarbons contained in six major reservoirs ranging in age from Early Jurassic to Late Cretaceous. The oil-bearing Mishrif/Rumaila limestone reservoir have been drilled and mostly appraised via wells dedicated to developing the Lower Cretaceous Minagish Oolite main reservoir which are estimated to account for up to 85% of the field's oil reserves. Mishrif is a tight layered, fractured reservoir of Cenomanian-Turonian (Upper Cretaceous) age developed over an asymmetrical anticline, dipping from east to west, the top of the formation was unconformable everywhere. Three main fault patterns were observed, defining three structural periods: EW and NS faults crossing the field in its central part during the Hith-Shuaiba period; mainly EW faults crossing the field in its central part during the Shuaiba-Ahmadi period; small NS faults in the north, EW trend en-echelon faults in the centre, NW-SE faults with a large extension in the south-west part of the field during the Ahmadi-Tayarat period. Most of the faults were generated during the Mutriba-Tayarat period (Turonian to Base Tertiary) but the trap formation of the Minagish field is post-Eocene. The recent Mishrif layering scheme divides the reservoir into 9 geological surfaces (time lines at field scale). There are two major sequence boundaries with exposure time, one at the top of the underlying Rumaila Formation and the other at the top of Mishrif (Fig.2). The lower part of the Mishrif consists of homogeneous mud dominated lithologies which is deposited in a slightly rimmed ramp environment. The upper part shows the occurrence of fine to very fine grained peloidal packstones to grainstones facies deposited in the more proximal shallower part of the ramp environment. The muddy sedimentation led to a very poor reservoir potential and the disappearance of porosity is due to heterogeneous cementation (nodules).