Abstract

Clays in reservoir rocks and non-reservoir rocks encountered during the exploration and exploitation phases interact with fluids and are responsible for numerous problems such as formation damage, excessive residual saturations, and wellbore instability. Such interactions are dependent on the composition of the fluids and clays in the matrix. Therefore. an accurate method for clay characterization is essential for efficient drilling. completion, and exploitation of oil and gas reservoirs. This paper presents a methodology based on thermal analysis which is able to characterize shales as well as other clayrich sedimentary rocks. The methodology is simple. inexpensive. and requires a small amount of sample. A description of the new methodology, its advantages and /imitations. as well as results with several rocks are presented. The suggested methodology is capable of distinguishing between free interloyer bound and structural water, that are unique to different clay minerals. and therefore it is able to analyze the effect of a given fluid composition in contact with a shaly-rock. in great detail. The methodology is suggested for screening drilling. Completion stimulation fluids and any other fluids injected in the reservoir during secondary and tertiary recovery operation. Further, the technique allows quantitative characterization of changes in reservoir rocks during any thermal recovery processes.

Introduction

Clay-fluid interaction has been studied for a long time, using both simple and sophisticated tests), with the aim of selecting the most compatible fluid composition to be used in the field. However, alteration of clay-rich rock composition after it has been contacted by a fluid are rarely reported2, even though this information is crucial to monitoring rock-fluid interaction. Lack of a simple and inexpensive methodology to characterize clays (including shaly-formations) might be the most important reason for the absence of such results. The technique normally used by the industry, X-Ray diffraction (XRD), is relatively expensive, requires special and sophisticated equipment, specialized personnel to operate and to interpret the results, and the results do not include characterization of the water associated with different clay minerals. This last limitation is, probably, the most important one. Water is an important component of clay minerals (being present in different forms: free, interlayer, bound, and crystalline), playing a decisive role in controlling the reactivity of a clay when in contact with a water solution. It has been shown that the water content of a shale, considering both its amount and distribution, actually controls the rock reactivity3,4.

Search for a simple quick, cost-effective and detailed characterization technique pointed towards thermal analysis. Thermogravimetry (TGA) provides detailed information about the rock's composition (including interlayer, free, bound, and crystalline water). so that it can be used in clayfluid interaction studies. The technique can be used both in the laboratory and in the field. using cuttings or cores. Clay alteration when in contact with different solutions can be monitored, providing a powerful method to screen fluids introduced in the reservoir. The methodology herein proposed has been available for quite a long time5–8.

This content is only available via PDF.
You can access this article if you purchase or spend a download.