Abstract

The Machar field is primarily a naturally fractured chalk reservoir; with the subsea wells requiring acid stimulations in order to be economic. This paper describes the design and implementation of an acid stimulation for a new intelligent well completion. The combination of a subsea development, a high rate boat based acid stimulation and a surface operated intelligent completion provided several challenges.

The stimulation challenges arose due to the long reservoir completion and the required use of flow control valves with small tubing within this section. This prevented the traditional use of ball diverters in order to effectively treat the whole reservoir interval. By using the isolation capabilities of the intelligent completion, several intervals were sequentially treated. Within each completion interval, limited entry perforating provided the method of diversion necessary to effectively treat each perforation. In order for this to be successful, the perforations had to be centralised and shot prior to running the completion. Limited entry perforating therefore required physical gun shoot tests and a pump test through the shot holes. The results and conclusions of these tests are included in this paper.

The completion challenges related to the stimulation included assurance of the integrity of the sensitive downhole electronics and hydraulics required to operate the downhole valves and sensors. This assurance was vital as, for the first time with an intelligent completion, the stimulation involved pumping concentrated hydrochloric acid down the well at rates of up to 50 bpm.

Implementation of the stimulation was achieved with the real time multizone downhole pressure and temperature data providing several fascinating opportunities to view downhole mechanisms. Of particular interest is the pressure transient impact of a stimulation on adjacent zones and the cooldown effect on partially trapped annuli. The analysis of this downhole data is presented.

Introduction
Machar Field

Machar is a subsea field (410 mmbbl STOIIP), draped over a salt diapir in the Central North Sea. The reserves are mostly contained in a naturally fractured chalk formation1. When stimulation re-opens communications with the natural fractures, the result is high productivity wells (Productivity Indices (PIs) in the range 130 to 600 bpd/psi). The pressure is supported through water injection. It is the fracture network which has led to the main Machar problem - water production. During 2000/1, water break-through occurred in all of the wells. Subsea logging work identified the high productivity fractures as the source of the water production. Water breakthrough was accompanied by high pressure, which rapidly killed the oil production from the wells and caused massive crossflow when the wells were shut in.

Previous Stimulations

During the exploration, extended appraisal and development of Machar, the stimulation design has evolved. The basic concept has however remained largely unchanged2,3:

  • Identify the natural fractures through a combination of recognising fracture mud losses and image logs (primarily dipole sonic).

  • Perforate only where there are natural fractures. Perforate at 2–3 shots per foot (spf) over a 6–15 ft interval on the low side only. Limit the total number of perforations to between 100 and 250.

  • Perform a high rate (40–60 bpm) boat based Mud Solids Remover (MSR) acid treatment.

  • Use 5/8" 1.3 s.g. ball sealers for diversion.

Fortunately, the acid stimulations on Machar have largely been successful and productivities have been maintained. This is attributed to the combination of the natural fractures and also the relatively hard nature of the chalk. This contrasts enormously with some other chalk fields in the North Sea where acid fracturing productivities have not endured. Fortunately for a subsea development, this means that proppant stimulations are not required.

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