Summary
Continuing reservoir management at mature stages often concentrates on delineating pockets of remaining mobile oil. This is becoming a major task for reservoir geologists and petrophysicists. Many old fields are coming up for reactivation as investment opportunities and there is an overall expectation that modern techniques can lead to additional recovery of between 10 and 20%. In this article we will discuss the screening criteria related to reservoir architecture, accumulation condition and production history.
The mobile oil remaining can be found in a number of predictable locations in reservoirs depending on their structural style and facies.
Attic oil along faults is perhaps the most simple configuration but sizeable volumes of remaining oil can also occur as a function of reservoir stratification and lateral discontinuity. A systematic overview of the different play types has been compiled based on structural or stratigraphic lateral continuity and vertical reservoir connectivity. Screening criteria have been derived on the basis of field examples and models for four play types.
The screening criteria specify minimum conditions which may lead to economic re-development with horizontal sidetracks from existing wells. In addition recommendations are given with respect to data gathering to confirm the presence of economically viable targets.
Introduction
Numerous oil fields that have been in production for many years are currently being reviewed to evaluate options for increasing their ultimate recovery. The task involves determination of the volume and location of remaining mobile oil and subsequently the technical and economic assessment of methods to recover this oil. The first part of this task is often difficult because of the poor quality of the data often associated with old fields. Nevertheless, certain basic data are usually available and the purpose of this article is to provide first round screening criteria based on these data in order to select those reservoirs for which re-development schemes are more likely to be economical. For the reservoirs selected, further study and some additional data acquisition will be warranted.
The data that may be expected to be present consist of well logs, limited core measurements, basic facies descriptions, original oil-in-place and cumulative production figures, structure maps and well positions. Having access to well completion data is also essential. Individual well performance data are often difficult to obtain.
The proposed screening scheme is based on a classification of the types of remaining oil configurations. Once such a potential oil pocket has been recognized, an attempt is made to assess its economic value by estimating a number of parameters with a limited degree of accuracy. Dip, original accumulation conditions, bedding thickness, reservoir profile, porosity distribution and original oil saturation can often be determined satisfactorily. More detailed reservoir architecture and particularly permeability distributions are more difficult to obtain.
The classification scheme for mobile remaining oil pockets consists of a division into reservoirs with either high or low vertical permeability/connectivity and a further subdivision into types with a high and low horizontal connectivity. In this article four major types of mobile remaining oil configurations, representing the four combinations of high and low vertical and horizontal conductivity, are discussed. The screening criteria presented are based on re-development with pairs of horizontal sidetracks from existing wells. A cost of $1,000,000 has been assumed per job for re-entering the hole, milling the casing and drilling and completion of the two sidetracks each of 300 m length. This is based on a variety of cost estimates obtained for land operations.
The economic analysis based on this method and on the cost level shows a remarkably large scope for re-development of reservoirs with oil rims, attic oil cases in faulted reservoirs and layer cake reservoirs with beds of contrasting permeability. Fluvial labyrinth type reservoirs1 are much more difficult to re-develop but a number of observations are made to suggest more favorable configurations.
Classification of Remaining Mobile Oil Configurations
The retention of mobile oil in sufficiently large volumes to allow economic re-development is largely controlled by the presence of heterogeneous pressure distribution and the fluid density and viscosity contrasts. This article is restricted to sandstone reservoirs containing light oil that have been developed with vertical wells and produced under reasonable draw-down conditions. In view of the potential for recompletion and infill drilling, the most important heterogeneities are faults, boundaries of genetic units, large permeability contrasts and baffles to flow such as shale intercalations. Following the subdivisions of clastic reservoirs into layer cake, jigsaw puzzle and labyrinth types one can already predict a number of typical oil displacement patterns.
By considering major large scale heterogeneities we can subdivide the reservoirs into types with a high vertical conductivity and those in which stratification and low permeable intercalations result in low vertical conductivity. Next we can make a further distinction between layer cake reservoirs with a high degree of lateral continuity of the beds and reservoirs where the lateral continuity is limited by faults or pinchouts of the sand bodies. This leads to the scheme shown in Fig. 1. To the first category, A, we can attribute oil-rim reservoirs with a high vertical conductivity in which unproducible oil columns are left between the vertical wells as a result of cusping and coning.
Poor lateral continuity can be formed by a normal fault (B1) which, even when nonsealing over the juxtaposed reservoir interval, traps oil in the up-thrown block against the caprock in the down-thrown block. Depending on the throw of the fault, the structural dip and the distance of the vertical wells from the fault, a volume of oil will remain when the well water runs out.
In labyrinth reservoirs one finds updip stratigraphic traps (B2) especially in low net/gross (N/G) cases. In such cases we also encounter poor sweep efficiency unless the well spacing is small (D). Poor sweep can also result from stratification with large permeability contrast between the beds, particularly when these are separated by impermeable intercalations (C). This situation is quite common in layer cake reservoirs. Even without impermeable separations crossflow may be limited if the vertical permeability of the low permeability layer is low. This situation frequently occurs in fluvial labyrinth reservoirs and this can occur in combination with configurations B1 and D.
