A New Coring Fluid for Vuggy Carbonates
Authors: Sprunt Eve S. , Wooten Susan O. 
Published: December 2007

Summary.
A salt-weighted coring fluid that contains a biodegradable cotton-fiber product, normally used to control lost circulation, was used to obtain 1,048 ft [319.4 m] of carbonate core from a well in the Arun field in northern Sumatra, Indonesia. The vuggy core was free of mud solids and required special cleaning only to remove salt deposits. This fluid should be beneficial in coring other vuggy carbonates, and modifications should make the fluid usable for both high-permeability and unconsolidated sandstones in which mud-solids invasion is a problem.

Introduction
Accurate core-analysis measurements cannot be obtained when core samples are invaded by mud solids. In vuggy carbonates, such as some portions of the giant gas-condensate Arun field, drilling mud enters the pore space during coring. Mud solids occupy PV that formation fluids occupy in the PV that formation fluids occupy in the reservoir, resulting in erroneously low porosity measurements. Mud solids may also porosity measurements. Mud solids may also decrease permeability by lodging in pore throats, and the clay minerals in the mud solids can distort special core-analysis measurements such as capillary pressure. Whole-mud invasion in the Arun lime-stone can be so deep that, in some cases, vugs in the center of 3.5-in. [8.9-cm] -diameter cores are completely filled with mud solids. For example, destructive tests showed that mud-solids contamination reduced the porosity of one 112-ft [34-m] interval of vuggy core by an average of 1.2 porosity units (p.u.). The average porosity of this core interval was 14.7 p.u. porosity of this core interval was 14.7 p.u. (ranging from 6.0 to 19.7 p.u.), and the median permeability was 119 md (ranging from 1.6 to 4,854 md). The problems with mud contamination were not associated with a single mud composition, but instead correlated with the degree of vugginess of the core. Vuggy cores obtained with a water-based polymer system were as badly contaminated as those obtained with an oil-based system. Techniques used to try to remove mud solids from the core samples included solvent flushes, ultrasonic agitation, water jetting, and phase-transition (ammonia) cleaning. These techniques removed only near-surface contamination, however, leaving mud solids in the interior of the core. Cooperative research between Mobil Oil Indonesia Inc., Mobil E and P Services Inc., and Mobil R and D Corp. determined that a salt rather than a barite-weighted coring fluid would be less damaging to the core and that adding the cotton-fiber product to this fluid would decrease dynamic filtration through the core. This paper describes how laboratory tests were used to investigate the mechanisms of whole-mud invasion and then used to test the new coring fluid that was designed to prevent invasion.

Invasion Mechanism
Analysis of Arun core indicates that the large vugs inherent to the formation create a problem. Bulk volume measurements are problem. Bulk volume measurements are erroneous because mercury easily enters the vuggy porosity. Compressibility measurements are in error because the jacketing material creeps into the pore space as a function of pressure. Porosity and permeability measurements are incorrect because the mud solids and ground formation rock collect in the vugs during coring. To evaluate core-cleaning techniques, samples of vuggy quarry rock from Austin, TX, had to be contaminated for use in laboratory experiments. Reproducing field contamination, however, was very difficult. Initially, clean, dry, vuggy cores were saturated at 1,000 psi [6895 kPa) with a 13.0-lbm/gal [1557-kg/m3] lignosulfonate fluid representative of that used in the Arun coring operation. This saturation procedure is considered "static" exposure of the core to the fluid. X-ray computer-aided tomography (CT) scans of these samples did not show the mud solids observed in cores from the Arun field. The vugs were contaminated by mud-solid residue only when the drilling fluid was thermally degraded at 340 degrees F [171 degrees C]. Thermal degradation was eliminated as the contamination mechanism because the Arun cores were never exposed to temperatures as high as those needed to produce mud degradation in the laboratory produce mud degradation in the laboratory experiments. Contamination of the vuggy cores by static exposure to the lignosulfonate fluid was abandoned and efforts focused on contamination by a "dynamic-filtration" process. In both drilling and coring operations, it is common practice to maintain higher pressure in the wellbore than exists in the pressure in the wellbore than exists in the formation to control the well (i.e., to prevent a kick or blowout). Fluid transfer from the wellbore to the formation during the actual drilling or coring processes, called dynamic filtration, is caused by this pressure differential.
JPT
P. 474