Prediction Of Permeability Trends For Water Injection In A Channel-Type Reservoir, Lake Maracaibo, Venezuela
Authors: van Veen F.R. 
Published: December 2007

Reservoir sands in the Eocene Misoa formation in Lake Maracaibo haze been interpreted as deposits of a river-dominated delta. In connection with a water injection project, a reservoir geological study of the N-2 channel sand complex was made. Permeabilities from cores could be related to grain size, sedimentary structures, and also to low-derived hydrocarbon filled pore volume, phi (1 - Sw). Permeabilities in uncored wells then could be estimated by means of their relationship to phi (1 - Sw). These permeability trends were mapped and used to select optimum locations for injection wells.

Introduction

The Eocene Misoa formation is the main reservoir in Lake Maracaibo, Venezuela. The sediments were deposited in a river-dominated delta of Mississippi size, which filled the rapidly subsiding Maracaibo basin, resulting in a sand-shale sequence of several thousand feet. Having already produced some 10 × 10-9 bbl from primary production, secondary recovery projects are becoming increasingly important. To projects are becoming increasingly important. To assist in planning a water injection project in the C-5 sand reservoirs in Lake Maracaibo's Block I Horst, a reservoir geological study was made based on a sedimentological analysis of 3,400 ft of continuous cores from three wells. The object of the study was to establish the external and internal geometry of the various sand bodies with a view to predicting permeability trends within the reservoirs and to permeability trends within the reservoirs and to optimize the location of water injectors.

SEDIMENTOLOGICAL ANALYSIS

The cores were cut lengthwise, photographed, and classified according to their lithology, sedimentary structures, and grain size. A subdivision was made into genetic units. Within these units one or more energy units can be recognized. An energy unit is defined as having the same bedding type throughout and a grain-size distribution that varies between narrow limits (Fig. 1). In the producible sandstones two main energy types were distinguished:high-energy sediments (dm-scale cross-bedded, horizontal bedded, or homogeneous), andlow-energy sediments (cm-scale cross-laminated).

An example of the subdivision of a genetic unit into two different energy units is shown on Fig. 6a. These energy units occur in a specific vertical order, dependent upon the depositional environment. In the example shown from the N-2 reservoir sand, a fine-grained dm-scale cross-bedded sandstone is overlain by a thinner very fine-grained cm-scale cross-bedded sandstone, forming a fining-upward unit. Such units have been interpreted as channel-fills in a lower deltaic plain environment.

INTERRELATIONS BETWEEN SEDIMENTARY STRUCTURES, GRAIN SIZE, PERMEABILITY, AND PHI (1 - Sw)

The plug permeabilities were averaged for each energy unit and plotted against grain size. The relationships are shown in Fig. 2. High-energy sediments are fine- to medium-grained and have average air permeabilities always greater than 300 md. Low-energy sediments are very fine to fine lower grained and have air permeabilities less than 300 md. Internal sedimentary features of a reservoir thus prove to be related to permeability.

In order to facilitate recognition of energy units in noncored wells, the electrical log properties of the two energy classes were properties of the two energy classes were subsequently studied. A convenient quantitative measure for each sand body was available for 174 Horst, Block I wells in the form of average hydrocarbon-filled pore-volume measurements, phi (1 -Sw). Because pore-volume measurements, phi (1 -Sw). Because porosity logs were not available from the majority of porosity logs were not available from the majority of wells, phi (1 - Sw) measurements were derived from true resistivity, Rt, according to an empirical correlation shown in Fig. 3.

For each interval in the cored wells for which phi (1 - Sw) had been calculated, the energy class was phi (1 - Sw) had been calculated, the energy class was established and the average permeability calculated.