A new formation-tester-while-drilling tool (FTWD) introduced to the Chirag field, in the Caspian Sea, in December 2003 has resulted in significant time savings. As of 1 December 2004, it had been run in two additional wells at Chirag, including the longest well with the greatest step out drilled to date in the Caspian region. Prior its use in this particular well, the tool had been qualified by comparison to wireline measurements which have established its ability to make accurate formation pressure measurements in the high-permeability formations of the field.
Use of the tool has involved improvement and optimization of test procedures to consider better depth control, the possibility of stuck pipe risk, and mud cake integrity.Recent Caspian experience has also included the use of a new 8-in. diameter multi-drawdown version of the tool at the Central Azeri field.
The Chirag field has been producing since 1997 and has been under water flood since 1999. To date, the Pereriv reservoir has produced over 300 million BO from 14 wells. Current oil production is 142 thousand BOPD, and water injection is 135 thousand BWPD.
The reservoir is in a very dynamic state, and FTWD pressure data are critical to reservoir management.Specifically, layer pressure data is used to tune the reservoir simulator, monitor injection response, and map dynamic fluid gradients in the field.
This paper focuses on experience from the use of GeoTap tool compared to wireline tester results at the subject fields including highlights of operational learning points.
Introduction
Formation-testing-while-drilling (FTWD) was introduced by Sperry Drilling systems' GeoTap* tool, in early 2003. This innovative pressure test tool was the first to incorporate a probe-based testing tool that can be combined with a standard logging-while-drilling (LWD) tool string. This technology enabled recording of formation pressures during the drilling process. It only requires a short pause in pipe rotation, about 10 min, so that the probe can be extended to perform a pressure test. However, as shown in this paper, experience in the Azeri and Chirag, Caspian Sea fields has resulted in improved testing methods. Although these testing methods may have only minimal impact on the drilling operations, they must be considered in the FTWD planning phase.
In the Chirag reservoir, wells using extended-reach drilling (ERD) were required; and this requirement necessitated the use of tubing-conveyed wireline logging. Tubing-conveyed wireline logging proved to be a costly option, with operational difficulties resulting in significant lost time. These difficulties prevented acquisition of all the pressure tests needed to identify potential production zones. The 2003 introduction of the GeoTap tool in this field resulted in significant cost savings and provided greater flexibility in pressure testing. From this initial success, the primary goal turned toward wireline replacement. As operational experience was gained, GeoTap was used in one of the longest ERD wells to date, without any incident or lost time.
The primary requirements for GeoTap to be considered as a wireline replacement are reliable measurement of formation pressures and fluid gradients in a reservoir. Confidence in the tool grew with each successful experience, and GeoTap data are now used as input to the full-field depletion plan. This enhanced role for GeoTap requires close quality control of the data by a reservoir management team. These efforts have resulted in a better understanding of flow barriers/baffles in the different reservoir units on both a local and regional scale. Recently, pressures-measurements have been acquired in the Balakhany units in the Chirag field to determine future potential of these units.
