Microstructural Damage In Cotton Valley Formation Cores
Authors: Strickland Frank G. 
Published: December 2007

Rock core samples are routinely used in the oil and gas industry to analyze in situ rock properties. In this study, we have examined the effects of coring on rock properties by studying the microstructures of cored samples from depths of 8500 feet to 9200 feet (2,582 M to 2,804 M) in the Cotton Valley Formation of East Texas. To characterize rock microstructure, we used to direct observations with a scanning electron microscope (SEM) and a petrographic microscope (PM) and high precision measurements (3% × 10(-6)) of strain as a function of hydrostatic pressure to 20,000 psi (137,880 kPa). The high precision strain measuring technique was recently developed and is termed "differential strain analysis" (DSA) by combining DSA data with direct microscopic observations, we were able to measure the cracks which are present at laboratory conditions but may not have been present at in situ conditions.

Our data show that crack porosity as high as 0.35 percent may be induced during drilling and subsequent percent may be induced during drilling and subsequent core excavation. Most of the induced crack porosity is associated with cracks which are aligned horizontally. The creation of these horizontal cracks may be due to unloading of the rock or to localized stresses induced by the core bit.

Although the porosity associated with induced microcracks is small, the permeability associated with these cracks may be large. Laboratory measurements of permeability at low confining pressure may be orders of permeability at low confining pressure may be orders of magnitude higher than actual in situ values.

Introduction

Rock core samples are routinely used in the oil and gas industry to analyze in situ rock properties. Laboratory measurements of porosity and permeability are often used to design stimulation treatments and to predict oil and gas yield. However, laboratory measurements of porosity and permeability may not reflect in situ properties because:laboratory experiments are usually performed at atmospheric or very low (1000 psi or 6895 kPa) confining pressure,drilling may alter the microstructure of the rock, andthe confining pressure on the rock core is reduced by as much as 15,000 psi (103.421 kPa) and the temperature is reduced by as much as 350 degrees F (177 degrees C) when the core is brought to the surface.

Some work has been performed to reduce the effects of external changes on the rock cores, but these methods are expensive and equipment is not readily available today (Ward and Sinclair, 1977).

In this study, we have examined the effects of coring on rock properties by studying the microstructure of cored samples from depths of 8500 feet to 9200 feet in the Cotton Valley Formation in East Texas. To characterize rock microstructure we use direct observations with a scanning electron microscope (SEM) and a petrographic microscopic (PM), and high precision petrographic microscopic (PM), and high precision measurements (43 × 10 (6)) of strain as a function of hydrostatic pressure to 20,000 psi (137,880 kPa). Microscopic observations are performed on thin sections which are specially prepared to eliminate the damage introduced by usual thin sectioning techniques. The high precision strain measuring technique was recently developed and is termed differential strain analysis (DSA). (Simmons et al. (1974) and (Siegfried and Simmons (1978).

From DSA data we can determine:the linear and volumetric compressibility as a function of pressure,the strain associated with cracks of a particular closure pressure, andthe orientation of cracks with particular closure pressures.

By combining DSA data particular closure pressures. By combining DSA data with direct microscopic observations we can measure the cracks which are present at laboratory conditions, but may not have been present at in situ conditions.

EXPERIMENTAL DETAILS

The Samples

The samples used for this study were selected from a forty-foot (12.8 m) long section of four-inch (10.16 cm) diameter core of the Schuler Formation in Panola County, Texas. The Schuler Formation is the Panola County, Texas. The Schuler Formation is the upper member of the Cotton Valley Group. The lithology of the Schuler Formation is quite variable; conglomerate, well to poorly sorted sandstone, calcereous sandstone, sandy limestone, crystalline limestone and various types of shale are found in the Schuler.