Uncontrolled water production in naturally fractured tight carbonate reservoirs with low porosities is a serious problem that confronts Petroleum engineers in Potwar region of Pakistan. The potential of hydrocarbons in these reservoir rocks is masked by the undesirable high water cut. Spectral Noise Logging, High-Precision Temperature Logging in combination with conventional production logging can yield valuable information on gas, oil and water flow profile and behind casing communication in wells.
This paper demonstrates how the (SNL-HPT) diagnostic tool was employed in a well producing from a naturally fractured carbonate reservoir to analyze flow stream to identify hydrocarbon and water flowing intervals and cross flows behind casing. Spectral Noise Logging (SNL) can provide information on reservoir flow units behind one or multiple barriers, which is beyond the spinner capability of PLT. The acoustic noise caused by moving fluid is the result of internal friction and normally audible in highly turbulent flows. Its spectrum and volume is strongly dependent on the fluid type, pressure, temperature and flow rate.
Well-A was drilled and perforated in Sakessar formation of Eocene age. The well flowed at low oil rate and high water cut (98%) despite being at an up dip location compared to a near-by producing well (Well-B) with less water cut from the same zone. To identify the problem, diagnostic logs (SNL-HPT) were run in combination in Well-A to ascertain the perforation's flow profile and flow behind casing if any.
The diagnostic logs interpretation indicated that the bottom most perforation sets were producing the major volume of fluid and no cross flow behind casing was detected. Based on the analyses of logs, the bottom perforation sets were plugged with calcium carbonate chips capped by cement plug. After water shut off job, the well was flowed and reduced water cut along with decline in gross fluid was observed indicating that the top perforations may have not been acidized. A small acid job was subsequently carried out to stimulate the upper perforation which improved oil rates and the water cut reduced to 60%. Later PLT was also conducted in order to acquire post water shut-off profile and verify SNL-HPT results.