Abstract

Estimations of reserves, especially by the volumetric method, is usually undertaken assuming water-wet conditions. However, in South East Asia, some initial reserves estimates have been grossly overestimated. One of the reasons for this, particularly in carbonates reservoirs of this region, is the particularly in carbonates reservoirs of this region, is the effect of different wettabilities. Reserves estimation is important at any state in the production cycle, but in frontier regions such as in South East Asia where initial production costs can be very high, care must be taken. This wettability effect can manifest itself in the form of 100% oil-wet or mixed or fractional wettability. Petrophysical data is of primary importance to the estimation of volumetric reserves, and an examination of different wettabilities on log and core results is undertaken. The effect of wettability on saturation exponent, cementation exponent and formation factor commonly overlooked until the reservoir production is mature. Capillary pressure and relative permeability can be changed quite substantially with different wetting phases, with subsequent influences on transition zone height, hydrocarbon-water contacts, producibility and residual oil saturation. These changes arse discussed with examples from different reservoirs. Reserves estimation is also undertaken by decline curve analysis, material balance, and at times, reservoir simulation. The effects of different wettabilities on these methods of reserves estimation are examined and presented.

Introduction

Wettability is defined as:

" the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids."

The wettability of a fluid/rock system is an integral component of the distribution and behavior of fluids in the reservoir.

Commonly, when a porous medium of uniform wettability that contains at least two immiscible fluids, one of these can be considered as the wetting fluid. In oil bearing reservoirs, the two immiscible fluids are usually oil and water.

Thus, in a static system, or the one that is in equilibrium, the wetting fluid will completely occupy the smallest pores and be in contact with a majority of the rock surface, if saturation is sufficiently high. The non-wetting fluid will tend to occupy the centers of the larger pores, and could form globules or "blobs" that extend over several pores (Figure 1).

Many engineers and geologists still believe that all hydrocarbon-bearing reservoirs are strongly water-wet, and most basic reserves estimations are based upon this misconception.

Reservoirs that exhibit some oil-wet characteristics have been described in the literature for over 50 years.

For example, Treiber, Archer and Owens in 1972, examined samples from Canada, Iran, Colorado, New Mexico, North Dakota, Oklahoma, Texas, Wyoming and Utah, and determined that 66 percent of the reservoirs were oil-wet. The authors have seen many examples of oil-wet characteristics in various parts of the world, especially in carbonates, although not exclusively.

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