Flow Diagnosis in an Extended Reach Well at the Wytch Farm Oilfield Using a New Toolstring Combination Incorporating Novel Production Technology
Authors: Lenn Chris , Bamforth Steve , Jariwala Hitesh 
Published: December 2007

A joint initiative between BP and Schlumberger began in mid-1994 to develop techniques for the diagnosis and treatment of unwanted fluid entries in high angle and horizontal wells. The first full scale test of the flow diagnosis technology to occur in a BP asset took place in an extended reach well at the Wytch Farm oilfield in November 1995.

In order to measure oil and water flow rates it is necessary to measure both the hold-up and velocity of each phase. A novel toolstring combination was developed which incorporated new sensors especially targeted at the stratified flow regimes encountered in horizontal wells and featured the capability of measuring flow parameters by several independent methods, enhancing confidence in interpretation.

The new sensors could independently measure the velocity of the oil and water phases by measuring the time of flight of injected passive fluid markers as well as by the well established oxygen activation method. Fluid holdup was obtained from a nuclear carbon-oxygen ratio method and an imaging tool. Additionally the imaging tool, based on an array of local electrical probes, could give a real time representation of the distribution of hydrocarbons and water in the borehole. Conventional production logging measurements such as the spinner, pressure and temperature were also made.

The primary objectives of the logging were to determine the oil and water flow profile along the cemented and perforated liner and monitor the movement of the reservoir oil-water contact. Entry points for oil and water as well as zonal contributions were confirmed by excellent correlation between the various logging sensors.

Introduction

Conventional production logging technology was originally developed for near vertical rather than near horizontal wells. Sensors such as the gradiomanometer lose effectiveness over 70 degrees deviation. Spinners and capacitance holdup meters are significantly affected by the flow regime and holdup profiles encountered in horizontal wells. The stratified flow regimes that are prevalent in horizontal wells require new measurement methods. Additional complications are introduced by certain types of completion such as screens and slotted liners.

This paper reviews the process leading to a selection of appropriate sensing methods and the development and testing of a toolstring with the necessary performance for effective flow diagnosis in horizontal wells.

A first goal for the new toolstring was to be able to give quantitative flow rate measurement of oil and water in a cemented and perforated liner, with the long term aim of being able to measure three-phase flow in uncemented liners.

The Sherwood sandstone oil reservoir in the Wytch Farm field straddles the coastline of Southern England. The onshore reserves were developed from multiple well sites using limited step-out wells over the period 1984 to 1990. After four years production and further appraisal drilling, development of the offshore reserves commenced using extended reach drilling. To date, ten offshore wells have been drilled, with step-outs of up to 8km with 2.7 km reservoir sections. The wells are all ESP pumped and oil production rates of up to 20,000 bpd have been achieved.

The reservoir monitoring strategy for the offshore wells identified a need to production log selected wells to assess flow profiles with respect to the reservoir zones and to monitor movement of the oil-water contact. This data would be used to help determine future well trajectories, optimise stand-off from the oil-water contact and design future well interventions (water shut-off, secondary perforating, etc.). The lack of suitable technology to assess flow profiles in highly deviated wells with two-phase flow was identified and BP Wytch Farm worked closely with Schlumberger in providing operator requirements to the project and providing wells for prototype tool trials.

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