DIAGNOSIS: A CASE STUDY
The new production logging tool discriminates between water and hydrocarbons (oil and gas) by means of four identical probes symmetrically positioned across the wellbore. Each probe is a bubble detector that yields one of two outputs, depending on whether it “sees” water or hydrocarbons.
Because of itsbinary nature, the measurement readily provides water holdup, without prior knowledge of water. oil or gas densities required for the calibration of a standard gradiomanometer. Unlike the gradiomanometer, the new measurement does not have to be corrected for friction effect or well deviation.
Because of its local nature, the new measurement provides an image of the distribution of segregatedfluids across the wellbore. This feature can help in understanding segregated flow regimes, especially in deviated wells. The bubble count (number of dispersed fluid bubbles observed per second) is very sensitive in detecting first oil entries and individual perforation contributions.
The use of the new measurement is illustrated by a field example where a leaking bottom plug was suspected to beresponsible for the well’s high water cut. It unequivocally identifies the source of water production where conventional sensors would have been inconclusive and provides the justification for a workover that resulted in blocking water entries and increasing oil production by a factor of three.
Production logging interpretation’ relies on the understanding of the movements of thefluids in the wellbore and of how these movements affect the logging measurements. The Digital Entry and Fluid imager Tool (DEFT) was recently developed in order to help understanding the nature of wellbore flow and improve confidence in production log interpretation.
The DEFT is based on local and digital sensors (probes), and is tailored to measure water holdup inmultiphase flow. The tool has 2 sections: the mechanical sonde and the electronics cartridge (Figure 1). It is compatible with Schlumberger production tools and can be positioned anywhere in the tool string. Auxiliary measurements include relative bearing which indicates the orientation of the probes, and a caliper which provides a single axis hole diameter. The DEFT holds 4 low frequency (LF) probesmounted on a 4-arm centralizer. Each probe is located on the inner side of a centralizer blade, and is thus protected when the tool is run in and out of the hole. The 4-probe configuration offers a fair azimuthal coverage of the wellbore in vertical, deviated and horizontal wells. The probes are easily removable for maintenance and transportation convenience.
The DEFT sensormeasures the Direct Current (DC) resistivity of the fluid surrounding its tip. The probe “sees” bubbles of hydrocarbon in continuous water phase or bubbles of water in continuous hydrocarbon phase. Since the ratio of the DC resistivities of hydrocarbon to salty water is very high (typically IO”), the probe output is highly contrasted depending on whether the probe “sees” hydrocarbon or saltywater. This contrast is attenuated in the case of fresh water and becomes insufficient to differentiate between hydrocarbon and water when salinity is as low as SOW ppm. DC resistivities of oil and gas heing of the same order of magnitude, the tool does not differentiate between these fluids and simply indicates hydrocarbon.
The probe is designed around a cylindrical conically tipped ceramicinsulator with an inner conductor (Figure 2, the probe is roughly the size of a matchstick). The probe sharp pointed tip is designed to minimize problems related to wcttahility and to remain clean in the harsh downhole environment.
Ideally, the probe pierces impinging drops without altering their shapes or trajectories. One would iikc this process to produce a “binary”...