Determination Of Irreducible Water Saturation Using Magnetic Resonance Imaging Logs (MRIL): A Case Study From East Texas, USAM
Authors: Oraby E. , Eubanks D.L. 
Published: December 2007

Determination Of Irreducible Water Saturation Using Magnetic Resonance Imaging Logs (MRIL): A Case Study From East Texas, USA

Control of water production and identification of pay zones with high irreducible water saturation are two formation evaluation problems in the United States, the Middle East, and other fields around the world. These problems have delayed completion decisions and have burdened field management with additional expenses. This paper discusses a case study from East Texas where key field problems such as these could not be solved with conventional logs alone.

In the subject field, water production is almost impossible to predict from conventional openhole logs. Some zones with relatively low water saturation, as calculated from the logs, produce with a relatively high water cut, primarily because much of the water is movable. However, other zones with high calculated water saturation produce water free hydrocarbons. The difficulty in predicting water production arises from the petrophysics of the producing formations, which are low-permeability, medium- to fine-grained shaly sands. Where grains are small, the formations have high surface-to-volume ratios that result in high irreducible water saturation and, thus, lowered bulk resistivities. Hence, the use of resistivity logs as pay indicators can cause low-resistivity pay zones to be overlooked and, consequently, net field pay to be underestimated.

In the last few years, nuclear magnetic resonance (NMR) logs have shown great promise in solving problems of formation evaluation that could not be directly resolved with conventional logs. The capability of NMR logs to differentiate between the immovable and movable water has added reserves of up to 10% in many fields where, although water saturation was high, much of the water was immovable. NMR logs have also been capable of providing better formation permeability than conventional logs, a feature that can save time and expense in well-completion decisions. Because of these two capabilities, it seemed appropriate to apply NMR technology to the subject field.

The tool selected for use in the field was an improved version of a mandrel-type, centralized, magnetic-resonance imaging device known as the MRIL@ C-type tool. A series of logs were run in several wells. Accurate determination of movable water from these logs allowed operators to (1) better select pay zones to minimize water production and (2) eliminate the expense of unnecessary conventional logs. The MRIL tool is now run as a regular service in this field. This paper includes examples of MRIL logs, discusses possible applications in the Middle East, and presents recommendations derived from NMR logging experience.

Introduction

In formation evaluation, induction and resistivity logs are the main hydrocarbon identifiers because of the resistivity contrast between high-resistivity hydrocarbons and low-resistivity formation brine and clay minerals. If, however, a clean zone exhibits low resistivity because of formation brine, these two logs are incapable of indicating water mobility. Because of this limitation, many potentially productive hydrocarbon zones with high irreducible water saturation are bypassed because of fear of excessive water production. A major problem in East Texas is identification of low-resistivity pay zones. Because of the very fine sand grains in this area, formation surface-to-volume ratios are large; thus, formations can hold large volumes of water.

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