Formation Pressure and Fluid Contact Envelopes Without Density-Neutron Logs: A Practical Workflow for Log-Poor Wells

Abstract

Some mature basin and marginal field wells are log-poor in the sense that they do not have density-neutron logs, and in some cases, no pressure information at all. Yet, one still needs to derive decision grade formation pressure and fluid contact (FC) estimates in order to derive UGRs for reserves, design completions, and manage risks in development projects. This paper describes a repeatable workflow for deriving FC envelopes in the absence of density-neutron logs, while keeping all assumptions visible and traceable. A new method of contact depth determination for conventional reservoirs utilizes four common types of data observed even in log-poor wells: resistivity behaviour and invasion indicators, pressure points and gradients (with actual MDT/RFT data used when available and indirect indicators used when no direct data is available), drilling and completion issues (such as losses, kicks, fluid shows and production test responses), and basic cross correlation with the offset wells. Unlike other contact depth determination methods, which yield a single contact depth at the wellbore, this method yields contact depth envelopes defined by P10/P50/P90 quantities that reflect the strength of the various pieces of information and constrained ranges that reflect the quality of the information and the likelihood of the stratigraphic configuration being correct. An assumption set approach is used to qualify the impact of uncertainties in cutoffs and saturation and to ensure that the contact depth does not reflect more than it should any assumption. Formation pressure is handled similarly. Pressure envelopes are generated from data points and gradient families related to fluid type and depth trends and the pressure/envelopes are constraint checked against drilling break, kick and stable mud weight windows. The resulting pressure and contact envelopes are transmitted into Volumetrics and are therefore used in STOIIP/GIIP estimates to accurately assess the true impact of uncertainty on hydrocarbon resources estimates. A "value of information" guide is generated illustrating which data points (limited pressure data, targeted logs or short test for example) would provide the highest benefit in terms of reducing the uncertainty of the resource estimate for the lowest cost.