Production Logging in Difficult Well Configurations
Authors: Ojukwu Kelechi Isaac , Edwards John Ernest , Cig Koksal , Chang Glenda 
Published: February 2016

Production logging is difficult in complex well designs such as horizontal multilateral (ML) wells with jagged level-two exits. The ML exits can obstruct the access of logging tools due to damage around the milled windows. Once inside a low-flow high-water cut reservoir, the challenge becomes measuring an oil inflow profile in stratified flow.

The access issue is worse in sub-hydrostatic wells completed with artificial lift. Horizontal gas lifted wells are easily choked by the presence of coiled tubing (CT). Y-Tools can be deployed in electrical submersible pump (ESP) wells to provide bypass tubing for logging strings necessary to log while pumping, although the system has not been successful in low rate wells due to re-circulation across the dynamic seal in the Y-Tool. This recirculation invalidates the log data and overheats the motor. As in gas lift completions, the low rate ESP completions also suffer rate reduction caused by the presence of CT.

This paper describes the methods used to overcome these access and production logging challenges in North Oman. A new tool configuration that comprises a vertical array of mini spinners and fluid type sensors was used to detect a 2% oil hold-up in a stratified flow. It has a surface controlled collapsible spinner cage that makes the tool slick to maneuver around obstructions in the well. This has overcome some of the level 2 bypass problems in ML wells. A temporary ESP and Y-tool upper completion has been used to log a 3-leg ML backbone successfully. To fix the recirculation problem, a modified Y-tool plug was designed to reduce the fluid leakage in the dynamic seal. A variable speed drive (VSD) at surface controlled the ESP motor speed, adjusting the pump rate to compensate for the rate reduction caused by the presence of the CT. Wireline conveyed tractors have overcome some of the choking limitations of CT in gas lifted wells. Openhole completions remain a challenge.

Introduction

There are numerous challenges for the well and reservoir management engineer who wants to acquire production logs (PLT) in brown fields. Often the wells were not constructed with intervention in mind, and as the water cut gets high it becomes a challenge to detect the oil at all.

Drilling methods and the understanding of multiphase fluid dynamics such as stratified flow have improved since the first horizontal wells were drilled in the early 1990s. Now we know it is best to avoid undulations in the trajectory of these wells to minimize water sumps and gas pockets, and to provide a straighter well for deeper reach with CT. Also, fluid entries can be masked by the hold up changes created by these deviation changes. Nowadays, several technologies have enabled holes to be drilled much closer to true horizontal, for example near bit continuous inclination and rotary steerable systems. However it is the old horizontal wells with undulations that have the high water cuts today, and are often the focus of well and reservoir management.

Multilateral wells were the next step in maximizing contact with the reservoir from one wellhead. Recent multilateral well designs may have fluid control devices on each lateral, with a production test of each leg possible from surface. However the early designs only have open hole branches, or milled casing exits into open hole laterals. In these wells even logging the backbone can be a challenge.

Openhole completions present another set of difficulties, especially if the well is not cleaned up at the time of logging as in Figure 1. The most difficult wells are long horizontal holes drilled in sub-hydrostatic reservoirs with a high water cut. These wells present significant access problems when there is insufficient fluid viscosity and velocity to circulate residual mudcake and shale debris to surface. The shale debris is released from water exposure or stress related washouts. This creates challenges for any logging device that has external moving parts such as spinners.

Reservoir pressure maintenance with water flooding in brown fields has led to an increased requirement for artificial lift due to high water production. As these wells become more sub hydrostatic gas lift is no longer enough to generate the required flowrates, hence more intrusive forms of artificial lift are installed such as ESPs, positive cavity pumps (PCP) and beam pumps (BP). Pump bypass systems have been developed for these wells to regain production-logging access, however these were not designed for low rate wells.