Interwell Communication as a Means to Detect a Thief Zone Using DTS in a Danish Offshore Well
Authors: Oftedal Arve , Davies John , Kalia Nitika , Van Dongen Hans 
Published: May 2013

Controlled acid jetting (CAJ) was developed for relatively thin but aerially extensive oil accumulations in low-permeability chalk formations. These relatively low-cost long horizontal wells enabled the waterflood development of a number of Danish offshore fields. On initial completion of the CAJ wells, large volumes of acid were bullheaded to remove the mud cake from the horizontal reservoir section. For the distribution of acid, approximately 4- to 5-mm CAJ holes were predrilled in the uncemented liner. However, in a number of the injector-producer patterns, short-circuiting of water was observed. In some cases, this was interpreted to have been exacerbated by the large-volume acid stimulations. Detection of the location of the thief zones is not straightforward in CAJ wells because of the restricted hydraulic communication through the CAJ holes.

This work presents interwell communication as a means to detect the location of thief zones in a CAJ well using distributed temperature sensing (DTS) technology along with a production logging tool (PLT) and water flow log (WFL) for the first time. The thief zones were located by alternatively producing and shutting-in a neighboring well (Well A2) and recording the temperature data in the injector well (Well A1). When the producer well was flowing, the temperature in the injector well stabilized during the shut-in period, instead of warming back at a constant rate. The temperature data from DTS collected during injection and shut-in conditions in the injector well was used along with the PLT and WFL logs to determine the location of the main thief zone at 9,278 ft. Another temperature anomaly in DTS data occurring at approximately 11,200 ft suggested the presence of a minor thief zone, potentially communicating with a third neighboring well (Well A3).

A thermohydraulic mathematical model, which was developed as part of this work to simulate the bidirectional fluid flow through the liner and annulus assembly, was used to determine the temperature signatures associated with thief zones in a CAJ well. It was discovered that a change in slope in the fluid temperature profile occurred at the thief zone when fluid was injected. Similar features were detected in the temperature profile obtained from the DTS log in the injection well. Hence, the model predictions concurred with the observations from the field.

Introduction

The CAJ well design was developed as a key enabler to exploit the relatively thin but aerially extensive oil accumulations in low-permeability chalk formations in the North Sea, as shown in Fig. 1 (Hansen and Nederveen 2002). These relatively low-cost long horizontal wells, up to 30,000 ft in length, enabled the development of the Dan West Flank and Halfdan oil fields, which otherwise would have been uneconomic. The initial decision to use ultra-long CAJ concept wells to develop these water flooded oil fields has, however, effectively dictated a zero-well-intervention policy in terms of inflow and outflow profiling, as well as the conformance treatment of thief zones encountered.

Although, the initial field developments have met expectations with respect to development costs and initial oil production rates, a significant number of waterflood patterns now exhibit faster than forecasted water-cut development, indicating suboptimal conformance. The injection profiles in certain waterflood patterns are not optimal due to natural and/or induced fractures acting as thief zones. The presence of thief zones negatively impacts ultimate recovery, which, in turn, has triggered the need to develop new CAJ well intervention technologies for detection and subsequent treatment of thief zones.