FLOW UNITS IDENTIFICATION AND CHARACTERIZATION OF HYDROCARBON BEARING ZONES, AQUA FIELD. NIGER-DELTA, NIGERIA.

posted on August 13, 2019

ABSTRACT

Reservoir characterization is the key to a successful oil field development program. The recovery efficiency of any reservoir is influenced by its heterogeneities; particularly the distribution of porosity and permeability. To develop a reservoir model that represents the reservoir properties we must be able to define the vertical distribution of reservoir properties “flow units” and each flow unit has its own characteristics which control fluid flow behaviour. The understanding of flow units allows us to identify preferential flow zones. This study involves the determination of the number and distribution of hydraulic units (flow units), key flow unit characteristics and qualitative interpretation of flow performance using Stratigraphic Modified Lorenz Plot (SMLP) and Flow Unit Speed (FUS) parameter in the three selected wells of “Aqua Field”, Niger-Delta, Nigeria. A total of forty-two (42) flow units were identified throughout the hydrocarbon bearing intervals of Wells B, D and E. Well B has sixteen (16) flow units with six (6) speed zones and ten (10) baffles zones. Well D has five (5) flow units delineated, with three (3) speed zones. Also a total of twenty-one (21) flow units were delineated for Well E with nine (9) flow unit speed zones; the rest being baffles. The number of flow units delineated is an indication of the extent of the reservoir heterogeneity. The dominance of the speed zones to the baffles zones in a hydrocarbon bearing interval is an important factor in the flow performance and recovery efficiency during enhanced oil recovery. These various hydrocarbon bearing intervals of “Aqua Field” are dominated by baffles and an anticipated long term production will be sustained by the baffles zones. This will be characterized by a steady to slow decline production behaviour. This study has shown that Stratigraphic Modified Lorenz Plot (SMLP) is a cost effective, quick look method of characterizing petrophysical flow units.

CHAPTER ONE

INTRODUCTION

1.1 Background Discussion

Petroleum geologists have long recognized the need of defining quasi-geological engineering units to shape the description of reservoir zone as storage containers and reservoir conduits for fluid flow. The exploitation plan for petroleum reservoirs is based on results from production forecasts, which are obtained from detailed reservoir studies. It is important to know how much oil, gas and water exist in place and how the fluids will move through the reservoir. In particular, fluid movement is difficult in highly heterogeneous reservoirs and considerable efforts to develop a good reservoir description that adequately defines the vertical and lateral distributions of reservoir properties are needed. Primarily the porosity and permeability distributions are considered. This is better understood when reservoir heterogeneity is better defined which has profound effect on all phases of hydrocarbon recovery. The variation of reservoir properties gives rise to different hydraulic (flow) units within a lithologic formation. A flow unit is a volume of total reservoir rock within which geological and petrophysical properties that affect fluid are internally consistent and predictably different from other properties of other rock volumes (Ebanks, 1987). A given flow unit exhibits similar permeability-porosity relationship and has similar properties for fluid flow. Flow units of a reservoir are relatively easy to determine using three parameters, porosity, permeability and bed thickness to generate cumulative flow capacity and storage capacity for crossplot (Gunter et al.,1997).The vertical variation in flow and storage capacity characterized by unique slope line give rise to different flow units which are indications of key flow unit characteristics such as conduits, baffles or barrier (Rushing and Newsham,2001).The flow unit division allows fluid flow within the reservoir to be better understood and categorized in a useful manner for simulation analysis and reservoir management.

1.2 Research Aims/Objectives

The Niger-Delta Basin has numerous marginal productive or abandoned oil fields. Typically production records are non-existent which hampers further development interest. The purpose of this study is to use a quick look method of characterization when data are limited. The permeability-porosity relationship within each flow unit allows us to identify and characterize the flow units using the Gunter et al. (1997) flow unit characterization method. However, without permeability distribution, flow unit prediction is not possible. This research seeks to predict flow units using only well log data when no core data and seismic data are available for analysis. The use of the core and seismic data will aid in refinement and validation of estimated porosity-permeability values obtained from well log and also the identification of fractures, faults and other structures which contributes or limits to the fluid flow potential of a reservoir.

By following the methodology the research aims to

identify the number of flow units within a reservoir using well log data;
predict the key flow unit characteristics in each hydrocarbon bearing zone;
determine the distribution of flow units within the hydrocarbon bearing zones; and
make a qualitative flow performance interpretation using the slope of inflexion and flow unit speed parameters.

1.3 Study Area/Tools

The Aqua-field is a mature field situated in the Central Swamp I Depo-Belt of the Niger Delta area (Fig.1a & c). Three wells, namely Well – B, Well – D and Well – E, used for this study were drilled to total depths of 11930 ft, 11600ft and 13132ft respectively (Fig.1b). The dataset used for this research is three conventional open hole well logging data which have Gamma Ray(GR),Caliper (CAL-X),Sonic transit time (ΔT),Bulk density (ρ_b) and Resistivity (R-ILD,R-ILS and R-MSFL), made available by Shell Petroleum Development Company (SPDC) of Nigeria.