Extending the Range of Multidetector Pulsed Neutron Logging and Nuclear Modelling Applications for Reservoir Surveillance in the Columbus Basin

Abstract

It is routine to perform through casing saturation logging in single casing string completions but moving into double strings has traditionally carried more uncertainty; especially using older saturation techniques such as sigma in fresher, lower contrast, water salinities and shaly sands. Published examples of Multi Detector Pulsed Neutron (MDPN) Saturation Logs from the Columbus Basin in Trinidad have demonstrated that high confidence gas saturations, in difficult sigma conditions, are routinely obtained through double tubing strings in hole sizes up to 12.25 inches. Recent logging work has extended the range of applicability to routinely run through two strings of pipe, and in some cases three concentric tubing strings, in large bore hole sizes up to 17.5 inches.

Nuclear modelling is used in intervention planning to confirm the viability of the planned surveillance, determine the nuclear attribute measurement precision required and predict the nuclear attributes response to key reservoir parameters such as Apparent Pore Density Change (APDC) used for decisions in recompleting the wells. Refining the nuclear attribute response relative to APDC prediction is a key enabler in differentiating and predicting the production profile of discrete flow units. This allows us to adjust the depletion strategy in the reservoir to maximize recovery.

Examples are shown indicating unexpected water overrunning gas from time lapse MDPN interventions. In another example the MDPN attribute response accurately and coherently responded to the formation gas saturation and clay volume, despite logging through three strings of pipe. The log confirmed the contact movement in one sand, residual gas saturation below the moved contact, undepleted gas in another sand, and validated the bottom water drive reservoir model.

Obtaining reliable saturations in these extreme logging conditions requires instrumentation and operating procedure consistency not typically applied with standard 2 detector tools. We will describe some of the innovative approaches taken to support the instrumentation in order to push the limits of MDPN applications into the conditions described.