When the water cut in one of its best-producing wells doubled in six months, operator Pemex (Petroleos Mexicanos) needed answers fast. The asset team was eager to figure out where the water was coming from, then develop and apply an appropriate water-conformance solution. However, there were several challenges to getting an accurate, reliable production log in an operating, horizontal well with multi-phase production flow. But without an accurate production profile, applying a water-conformance treatment was riddled with uncertainty. Pemex partnered with Schlumberger, to do a thorough study of the problem, which included using the service company's fiber optics-enabled, integrated coiled tubing (CT) production service to deploy its multiphase horizontal and deviated well production logging system, to get the required flow profile of the well’s entire producing zone. The study results and log data analysis helped Pemex make the right decision, to avoid serious production losses in the field's most prolific well.