Subsurface Appraisal and Field Development Planning of the Gas Condensate Field GVLA
Authors: McKay David Maxwell , Wilcox William , Yang Anping , Liang Baosheng , Brinkman Joseph Patrick , Ginger Elliott , Balasubramanian Sriram , Wang Ben , Li Yan , Ogden Katharine Anne , Kennedy Daniel 
Published: May 2013

Abstract
The Greater Vanza Longui Area (GVLA) field is located in Area B of the offshore Cabinda, Angola Block 0 concession. GVLA is a multi field development and will target hydrocarbon resources in the Cretaceous Pinda formation that contain reservoirs with rich gas condensates and underlying volatile oil rims.
The objective of this paper is to demonstrate the rigorous subsurface evaluation process employed for developing a large offshore gas condensate field. The strength of the reservoir evaluation is the large amount of good quality appraisal data gathered and used to benchmark subsurface inputs. There have been 6 reservoir penetrations, 4 DSTs and 3 full reservoir cores and multiple fluid samples.
A structured workflow was followed to identify and incorporate the uncertainties in the subsurface assessment. Probabilistic forecasts helped to fully characterize and realistically assess the uncertainties. The uncertainties were grouped mainly into static and dynamic categories.
The impact of the static uncertainties was assessed using 3-D geological models. Geologic models were built upon a core-based sequence stratigraphic framework for the Pinda formation. The facies, petrophysical properties and top depth of reservoir were varied to capture the uncertainty ranges seen in the GVLA wells and analogue fields. These models captured the net-to-gross uncertainty which has the largest impact on hydrocarbons in-place and connectivity.
The dynamic uncertainties were assessed using statistical design of experiments. Monte Carlo simulations were employed to generate the probabilistic estimates. Reservoir simulation with the compositional formulation was used as the primary forecasting method. Robust equation-of-state models were built to appropriately quantify natural gas liquids (LPGs) which were value drivers for the project. Various development alternatives (primary depletion, gas injection) were evaluated and detailed economic analysis were performed for concept selection. The production forecasts were verified with material balance models, analogues.

Introduction
The Greater Vanza Longui Area (GVLA) is located in the western portion of Block 0 of the offshore Cabinda Province, Angola. The GVLA project area is approximately 12km by 12km, and ranges in water depth from 300ft to 600ft. Sonangol, Total and Eni are partners in this project with Chevron as the operator. The project area consists of five adjacent fault blocks (Fig. 1): Longui Main, Longui East, 79-B, Vanza, and 80-E, all of which have been grouped into two main development areas, Longui and Vanza. The GVLA oil and gas reservoirs are contained within the Cretaceous Pinda Formation (Fm.). The GVLA development is an attractive economic opportunity that will fill projected gas supply for the Angola Liquefied Natural Gas Plant (ALNG) project. The recommended recovery mechanism is primary depletion of the gas cap and oil column, with gas/gas liquids processing at Sanha FPSO and ALNG.