Improved Production Log Interpretation in Horizontal Wells Using Pulsed Neutron Logs
Authors: Brady J.L. , Kohring J.J. , North R.J. 
Published: December 2007

Production log flow profiles provide a valuable tool to evaluate well and reservoir performance. Horizontal wellbores and their associated completion designs present several challenges to profile interpretation for conventional production logging sensors and techniques. A unique approach combining pulsed neutron capture (PNC) log data with conventional production logging measurements is providing improved flow profile answers in slotted liner, horizontal well completions on the North Slope of Alaska.

Identifying and eliminating undesirable gas production is one of the chief goals of production logging on the North Slope. This process becomes difficult in horizontal wellbores as fluid segregation affects the area investigated by the various logging sensors and also the velocities of the individual phases. Typical slotted liner completions further complicate analysis as fluids are able to flow in the liner/openhole annulus.

Analysis of PNC log data provides two good qualitative indicators of formation permeability. The first technique is derived from the difference of the formation sigma response before and after injecting a high-capture cross-section borax solution. The second technique uses the difference of the formation sigma response and the formation porosity measured while injecting the formation with crude or seawater. Further analysis of PNC log runs show that the two techniques closely correlate with production flow profiles under solution gas-oil ratio (GOR) conditions. These two techniques in combination with conventional production logging measurements of temperature, capacitance, pressure, and spinner improve flow profile results. PNC results can be combined with temperature and pressure data in the absence of valid spinner data to provide an approximate flow profile. These techniques have been used to successfully determine profiles in both cemented and slotted liner completions with GORs in excess of 15,000 scf/bbl.

Introduction

The determination of downhole flow profiles is an important part of reservoir management. Beginning with an analysis of the well's initial completion efficiency, these profiles provide zonal production allocation and analysis of break-through and coning events and are used to check the the mechanical integrity of completions. Production log surveys are commonly run as problem diagnostics when anomalies in surface production rates are noticed. In these cases, the survey results are used to plan remedial actions to bring the well back to its optimum producing efficiency. These answers are proving to be particularly important for horizontal wells on the North Slope of Alaska.

Figure 1 shows a typical North Slope horizontal well drilled to produce in a gravity drainage depletion type of reservoir. The cross-section shows that the horizontal well is placed as deep as possible below the gas-oil contact (GOC) while still remaining in producible reservoir rock above the formation base or the aquifer. Although the GOC is generally 50 to 100 ft. above the horizontal wellbore, undesirable gas production often occurs. Possible sources of this gas production include channeling down the casing/borehole annulus through poor cement isolation, coning through higher permeability sections of the reservoir, gas being sucked down permeable fault planes or some combination of the above. It is not unusual to have a gas under-run directly below a shale.

Conventional Production Logging.

A typical, conventional production logging tool string usually includes spinner, capacitance, fluid density, temperature, and pressure sensors. The spinner is used to measure the total velocity of the fluid mixture, and the capacitance, density, and temperature tools are used to determine the instantaneous holdups of water, oil, and gas.

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