Abstract

At present, hundreds of horizontal wells have been drilled in Hassi Messaoud counting for about one fourth of the field's output. Among these, several wells are underperforming and could not yield the expected production. Vertical and areal permeability anisotropies, absence of natural fractures, severe heterogeneity, reservoir damage and wellbore instability are the main factors to blame for poor productivity. To address the problem of poor vertical communication and get past the near wellbore damage, hydraulic fracturing has been thought of as the best solution to improve productivity.

This paper describes the candidate selection criteria, the design and execution of the first fracturing job to be carried in a horizontal well. A new openhole completion system is to be deployed in the horizontal section. The system uses a series of mechanical openhole packers that allow for stimulation in each desired interval. The use of mechanical packers provides positive mechanical diversion at high differential fracturing pressures. In this case the horizontal drain has been divided into six sections. Three of them have been hydraulically fractured while the remaining sections characterized by natural fractures are producing naturally by simply opening the frac port. The system has been designed, so the fracturing treatments along the horizontal wellbore can be pumped in one continuous operation, thus minimizing the associated risks and optimizing the efficiencies of both personnel and equipment needed to perform the work. In this case the pumping operation has been successfully performed in less than 48 hours with no operational issue. The first operation has proven the feasibility to set multiple hydraulic fractures in openhole horizontal wells in Hassi Messaoud field. The drastic production increase demonstrates how the multiple fractures allow improving the vertical and areal drainage of the reservoir for this type of wells.

The article presents all the project phase from candidate selection, design to job execution. It also highlights the risks involved with regard to many aspects during the project. Finally, the paper summarizes some lessons learned and draws suggestions for prospective candidate wells.

Introduction

The Hassi Messaoud reservoir has been on stream since 1958. Currently, it produces about 65 Mm3/D of 45°API oil from a thick Cambro-Ordovician sandstone formation. The structure is a flattened anticline with a sequence of horts and grabbens contained by faults in a submeridian direction. The faults are generally oriented SSW to NNE and cross the entire reservoir. The depositional environment is fluvial at the bottom to marine at the top. The shaly sandstone contains silt interbeds which have a thickness between 0.2 and 0.4 m with a metric to kilometric lateral extension. The reservoir is naturally fractured in some parts because of tectonic activity. Fractures are either open or plugged with materials such as shale, silica, anhydrite, pyrite, and bitumen.

The reservoir is subdivided into four distinct formations—Ri, Ra, R2, and R3—in addition to a zone of alternance with Ra-Ri and R2 being the main and secondary reservoirs respectively. The producing Ra-Ri is subdivided into six intervals: D5, D4, D3, D2, ID, and D1. The reservoir has a porosity ranging from 6 to 12 %. Overall, the permeability is low with a range from less than 1 to more than 100 md in open-fissures layers. The reservoir was initially undersaturated with an oil saturation of about 80%.

The reservoir was subdivided into productive zones. Most zones are under miscible gas flooding. Five zones are under water injection. Up until 1995, the reservoir was developed using vertical wells. Along with matrix treatments, hydraulic fracturing is extensively used with more than 300 wells already treated to compensate for low permeability or to bypass nearby wellbore damage. Gaslift is used for production activation throughout the field.

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