Abstract

The following is the presentation of two field examples of NMR logs in carbonate reservoirs with low porosity and permeability. The wells were drilled with WBM in Devonian (D2-D3) carbonate sequences at Orenburg district (Russia). Conventional open-hole logs fail to recognize pay zones. The NMR measurements allow identification of vuggy and inter-granular porosity; however, several problems occur with calculated permeability. Presence of clastic "tongues" in fore-reef facies provides multiple challenges for advanced formation evaluation. Core data suggested presence of oil-saturated reservoir quality rocks with subsequent testing indicating commercial flow. Modified version of Coates permeability model have been used to match the testing results.

NMR method should not be used as a replacement for any porosity logs in reservoirs with mixed lithology. Modern tools provide spectral porosity distribution that includes signal from hydrocarbons. Interaction of the formation fluids with pore surfaces can significantly influence relaxation response hence, a good knowledge of reservoir geology and lithology is a requirement for interpretation in carbonate reservoirs.

Introduction

Assessment of productivity in carbonates from well log data traditionally presents challenging and complex problem (3,7). Most reliable information about permeability comes from well test results performed by formation testers and pressure transient analysis (8). NMR technology has made a great impact on open-hole formation evaluation; and it has also receives a lot of professional attention, as well as advertising. Unfortunately, benefits of NMR application in carbonate reservoirs are coming hand-by-hand with serious danger of potential misinterpretation. Often it leads to faulty interpretations or other mistakes commonly blamed on the method. Although suggested for a long time as a "lithology-independent porosity" and as a "unique permeability measurement" NMR in reality appears to be neither. T2 interpretation remains uncertain in the case of mixed lithology, were none of previously suggested cutoff values produce a reliable results, or when geological conditions break a correlation between surface-to-volume ratio and permeability.

In 2001–02 this technique has been successfully applied in several sedimentary basins in Kazakhstan and Russia (Orenburg district), where complex predominantly carbonate sequences contretemps conventional logs interpretation. Vugs, which are defined as pores larger that adjacent grains (3), are common here, but not readily recognized on conventional data. The main purpose of this study was to describe the experience that was gained so far, and spot and indicate limitations of current NMR technology illustrating them with two field examples.

GEOLOGY AND GENERAL SETTINGS

Various reef mounds are pretty common across the flanks of Kamsko-Kinelsky Depression (Orenburg District, Russia). Typically, four major zones of deposition are recognized there: back reef, reef flat/crust, front and fore reef. Within those zones up to ten various lithofacies could be identified on cores and logs. Presence of clastic reservoirs on the external side (fore-reef) was noticed in several oil fields. Some contradiction exists on their origin; whether they were formed by active channels running seaward between the reefs or were a result of shallow basin turbidite deposition (fig 1).

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