Productivity Preservation Through Hydraulic Propped Fractures in the Eldfisk North Sea Chalk Field

Summary

The Eldfisk oil field is a high-porosity, low-permeability reservoir of soft chalk in the Norwegian North Sea. Commercial oil production from the Eldfisk field is dependent on well stimulations. To date, pseudolimited-entry acid stimulation (PLEAS) has been the standard completion technique. This technique initiates relatively high (5,000 BOPD) individual-well oil-production rates. Over time, however, many of the Eldfisk wells have experienced rapidly declining oil production characterized by wellbore skin increase and fracture closure. Solids production and casing collapse have also been problematic. In late 1997, an alternative completion technique involving four hydraulic propped-fracture stages was successfully executed in an Eldfisk horizontal well. This paper presents a simple methodology to evaluate sustained productivity enhancement of propped fractures vs. acid stimulations in soft chalk formations based on the use of analytical modeling in combination with empirical production-data analysis.

Introduction

Since the early 1970s, petroleum engineers working on North Sea chalk fields have been challenged to design well-stimulation treatments focused on hydraulic fracturing of fragile chalk formations. Such stimulations are necessary to get the low-permeability chalk formations to produce economically. Conventional acid stimulations have been tried on all the chalk fields. Both field experience and research¹ have shown that acid stimulations render attractive near-term oil production; however, for some of the chalk fields, the medium-term production period is poor. The falloff in production is attributed to low hardness and the homogeneous nature of the chalk, whereby the near-wellbore cavities and etched fracture surfaces created by the acid stimulation collapse with increasing stress.

To date, there has been oil production from more than 10 chalk fields in the North Sea. Geographically (starting with the southern North Sea Danish chalk fields), after starting with acid-stimulation treatments, there have been more than 100 propped-fracture treatments placed in horizontal wells in the Dan field.² Offset to the Dan field is the Gorm field, which also has used hydraulic propped fractures in some of its water-injection wells.³ Further north, still in Danish waters, the South Arne chalk field-development strategy has focused on the drilling of horizontal wells with completions involving multiple propped fractures.⁴ Propped-fracture stimulations in Norway were introduced successfully at the Valhall chalk field.⁵ The evolution from exclusive use of acid stimulations to propped fractures for the referenced fields has centered on one mutual goal, namely productivity preservation.

For the four oil-producing chalk fields (Edda, Ekofisk, Eldfisk, and Tor) located furthest north in the North Sea - deemed the Greater Ekofisk Area - it was concluded that for most cases, the optimum stimulation technique is PLEAS.⁶ This conclusion was based on studies that indicated that propped fractures would not be successful because of excessive proppant embedment in soft chalk. However, in 1997, a propped-fracture "pilot" was pumped in an Eldfisk well to help further the understanding of efforts toward individual-well productivity preservation. This paper describes the Eldfisk declining-productivity problem and its symptoms and examines the root causes of the problem. A possible remedy to the problem based on the pilot performance for a well with hydraulic propped fractures is reported.

Eldfisk Field Background

The Eldfisk field is located in the southern section of the Norwegian North Sea, in Block 2/7 (Fig. 1). The Eldfisk field was discovered in 1970 with the drilling of Well 2/7 1X. Estimated field fluids in place are 2.8 billion bbl of oil and 4,600 Bscf of gas. The initial stock-tank oil gravity was 37.3°API. The undersaturated oil had an initial pressure and temperature of approximately 6,800 psia and 268°F, respectively, with an R_s value of 2,590 scf/STB and a viscosity of 0.11 cp. The field came on production in 1978 and was on primary production until the year 2000, at which time a waterflood was initiated in addition to gas injection of excess volumes above transportation capacity. Through the year 2000, the field produced approximately 440 million STB of oil. It is estimated that 30% of the oil produced to date is from the rock-compaction mechanism, 30% from gas influx, 20% from oil expansion, and 20% from water influx.

Well Stimulations.

Over the past 30 years, many new drilling and completion techniques have been tried in the development of the North Sea chalk fields. However, stimulation design has not changed considerably at Eldfisk since 1985. A standard treatment includes alternating stages of gel (200 to 300 bbl), 28% hydrochloric acid (150 to 250 bbl), and overflush (100 to 150 bbl). Diversion is achieved with 1.3-g/cm³ ball sealers. Today, the Eldfisk reservoir, as with all the Greater Ekofisk chalk reservoirs, is stimulated with massive, high-rate, multistage PLEAS treatments. The perforating technique involves clustering perforations in 2- to 3-m intervals, where each interval is separated by 10 to 12 m. As explained by Snow and Hough,⁶ a limited-entry design is one in which the perforations are limited to create high friction pressures in the perforations. The perforations behave like a backpressure valve, ensuring that high pressures are applied across all perforations and that breakdown of the entire interval occurs. The PLEAS technique has proved highly effective for the more consolidated chalk reservoirs. However, for the chalk fields with the highest initial porosities, acid stimulations are often destructive to the medium-term vitality of individual wells.

The Problem and Its Symptoms

The main problem addressed here is the rapid decline of production in many Eldfisk wells following acid stimulations. Symptoms of the problem include casing deformations and solids flow in individual wells.