Interpretation of reservoir boundary conditions and well drainage areas have been historically done conventionally; using analytical and numerical simulation approaches with constant pressure, no flow, leaky and conductive boundaries. This paper investigated the effect of Gas Water Contact (GWC) boundary on the pressure transient behavior. The novel approach adopted involves building numerical simulation well test models that investigate the effect of using Carter Tracy analytical aquifer model to simulate the response at the aquifer interface of a gas reservoir using SAPHIRE software. Bourdarot1 and Kuchuk2 stated that the effect of an aquifer can be modeled with a constant pressure boundary model. This model assumes that the pressure at the boundary of the reservoir consistently remains at the initial reservoir pressure during the drawdown and build up phases of the well test. Hence, it suggests that the pressure support from gas cap is very strong due to expansion and that the multi phase flow effects can be neglected. These assumptions work well for gas cap depletion systems. However, in the case of a water drive system, they may be incorrect. The result of this investigation indicates that pressure transient at Gas Water Contact boundary behaves like a constant pressure boundary for gas reservoir with small sized aquifer or a radial composite system for gas reservoir with large or infinite aquifer respectively. The former is due to the change in fluid diffusivity and very high mobility contrast. (Khμ)1→(Khμ)2 (with 2>>1) coupled with expansion of active gas cap while the later is due to mobility contrast coupled with expansion from the active water influx. The result of this investigation was compared with the conventional analytical method of using a constant pressure boundary assumption. It is recommended to apply the results of this investigation in: (i) estimating the Gas Water Contact in a down-dip reservoir and therefore help in the quantification of reservoir volumes to support existing reliable technology for determining the Lowest Known Hydrocarbon (LKH) in a Gas Down To (GDT) scenario by applying shrinking box technique to a recognized onset of constant pressure or radial composite effect in a DST or multi rate test in a down dip gas well (ii) Analytical Aquifer boundary modeling in well test designs as it affects drainage area and volume.

Net to Gross (N/G) is a key input parameter into the estimation of Hydrocarbon In-Place volume. Errors in N/G evaluation could result in huge impact (up to 40%) on estimated in-place volumes which would in-turn affect the economics of reservoir/field development. Traditionally Gamma ray (GR) and/or SP logs are used for delineating reservoir sands from non-reservoirs. However, in some instances, these logs are not able to differentiate fully some reservoirs from non-reservoirs. In the case of GR, it is not able to resolve differences between shales and sands with high radioactive materials contents. Both GR and SP logs do not differentiate non reservoir cemented tight sand formations. This lack of contrast on the logs in such circumstances results in errors in the estimated net reservoir sands, leading to inaccuracies in the overall estimated reserves. To account for the deficiencies of these traditional techniques, a combination of neutron porosity and formation density logs may be deployed. These tools are able to provide contrasting petrophysical properties that differentiate between impervious formations and reservoir sands, thereby providing robust technique for improved net sand estimation. Incidentally, most old wells drilled in the 1950s up to mid 1980s do not have the neutron and the density logs. Yet for some early operators, a majority of the old wells were strategically positioned that the openhole data from them would be very crucial for estimation of reserves in the concerned fields. A cursory review of the log database reveals the abundance of Micrologs (Microlog Normal (MLN) and Micro Log Inverse (MLI)) in most of the early wells. Careful re-interpretation of these logs using appropriate techniques and principles have resulted in significant improvement in net-to-gross estimation in the absence of neutron and density logs. This paper aims at re-establishing the resourcefulness of microlog data in error reduction on estimated net to gross with a case study in ELJODA Field, Niger Delta Nigeria.

This work demonstrates the usefulness of data quality checks for the purpose of achieving test objectives with an example from a Niger Delta well. The well UGO-1X was completed as single-zone single string (SSS) configuration with a 4-1/2 inch, 12.75 ppf, 13Cr HCS production tubing. The well was tested in order to characterize the reservoir, determine the completion efficiency and ascertain reservoir limit for GIIP estimation. The test program involved multirate production, followed by a build-up phase for which a Down-Hole-Shut-In-Tool (DHSIT) was deployed to manage wellbore storage effects. However with the conclusion of the multirate test, and commencement of build up, the downhole shut in tool (DHSIT) failed and subsequently the well was shut-in at the surface and the build up (BU) stage allowed to progress as per programme.Following the conclusion of UGO-1X multi-rate test (MRT drawdown and build up), the data was retrieved from the quartz gauges, quality checked and analysed using the conventional and numerical simulation methods. This paper illustrates the difficulty of interpreting an incomplete set of data, the importance of properly understanding the operational history in well test analysis as well as the usefulness of conducting a quick analysis to validate data thereby avoiding a repeat operation. It is shown that by careful reprocessing of the data (de-listing data within the DHSIT failure interval), the overall quality of the data could be significantly improved and used to produce credible results. This made it unnecessary to conduct a repeat of the MRT/BU on UGO-1X as initial test objectives were achieved.