Perforating in Overbalance - Is It Really Sinful?

Summary

Drilling and completing long horizontal wells in North Sea oil fields is extremely costly. When perforating these long horizontal sections (>1000 m), deployment of the perforation guns on drillpipe has been regarded as the safest and most cost-efficient method. The well is then kept in overbalance until the completion is installed.

To maintain well control, a fluid with loss-control capability [kill pill (KP)] is used as completion fluid during perforating to seal the formation immediately after perforating. This saves the time required to displace completion brine and the cost of losing the expensive brine into the formation. However, perforating with the kill fluid in the wellbore presents a great challenge in minimizing formation damage. Therefore, optimizing the perforating design and choosing the proper kill fluid are crucial tasks to ensure good well productivity. A thorough laboratory testing program has been performed to re-evaluate perforating strategy and completion-fluids selection for several fields in the North Sea. The fluid systems investigated include oil-based mud and water-based KPs formulated from formates and bromide brines.

The laboratory results have shown that if the perforating is designed and executed properly, formation damage can be reduced to a minimum even during overbalanced perforating. In this work, the major damage appears to be from the kill fluids. When using a water-based KP, the reduction in relative permeability appears to be an important damage mechanism. Therefore, the productivity is influenced strongly by the fluid-loss-control capability. In contrast, filtrate invasion from the oil-based mud does not alter relative permeability; therefore, the formation damage shown in these tests is less pronounced.

Furthermore, zinc- and steel-cased charges have been used to investigate their compatibility with kill fluids and their impact on formation damage. These studies have shown that interaction between zinc and CaBr 2-based KPs can cause failure of fluid-loss control and excessive invasion of filtrate, leading to severe productivity impairment.

This paper presents some of the results of the laboratory testing program.

Introduction

Completing a horizontal well in a hostile environment such as the North Sea is a complex and costly operation.

To reduce risk and cost, Hydro has (for some fields) practiced overbalanced perforating. A perforation fluid containing viscosifier and fluid-loss-control material is spotted across the reservoir section before perforating. A filter cake is created immediately after detonation to minimize fluid loss and formation damage. The well is then shut in with the kill fluid in place for several days while the remaining completion is installed.

The perforation fluid is optimized for the specific field or well; however, brines containing bridging material and polymers are most often used. The type of brine is selected on the basis of the specific gravity requirement and the formation-damage potential.

In this work, perforating fluids based on heavy brines such as KCOOH, CsCOOH, and CaBr2 are presented. Oil-based-fluid systems also are investigated.

Zinc-cased shaped charges have been used to perforate long horizontal wells because zinc shatters into fine powder-like perforating debris, which is easy to mobilize and produce out of the well. If necessary, the debris can be removed by an acid treatment, although this is not standard practice for Hydro. It is assumed that little perforation debris remains in the well after cleanup, and the risk of obstruction for future well interventions is therefore minimized.

However, it has been observed in the past1-3 that the zinc-charge debris could react with a calcium-containing water-based fluid to form precipitants and cause problems in well-completion operations. Concerns were also raised that these precipitants might severely plug the formation around the perforation tunnel.

Minimizing perforation damage requires proper design of the perforating job. There have been many studies conducted to optimize perforating design with completion brine in the wellbore. Most concluded that underbalanced perforating is necessary to prevent high perforation skin.4-7 Recently, new studies have shown that transient underbalance plays a more important role in perforation cleanup than static underbalance. 8-10 This transient underbalance can be generated even in overbalanced perforating if carefully designed. In this paper, the additional challenge is to optimize overbalanced perforating with a drilling mud or KP in the wellbore. An extensive study was conducted to characterize formation damage caused by various drilling fluids and KPs when used in overbalanced perforating.

The experiments were designed to closely simulate field conditions when perforating overbalanced in a well open to the trip tank. Information about the reservoirs, the completions, and the perforating practices for the two fields studied in this test program are given in Table 1. The reservoir parameters such as pore pressure, overburden stress, and bottomhole temperature and the well parameters such as casing diameters and perforating design are incorporated into the experiments.

Fluids

The careful formulation and quality control of wellbore fluids is important for the field conditions to minimize formation damage. This becomes even more important in a laboratory study in which the wellbore fluid is one of the primary variables and reliability and reproducibility of results are paramount. For this study, all the fluids were formulated, mixed, and quality controlled at the operator's research center in Bergen, Norway, in close collaboration with their supplier of drill-in and completion fluids for North Sea operations. The fluids were shipped in timely batches to the supplier's Houston facility, where a repeat quality control was performed. Once approved, the fluids were forwarded to the experimental test facility for immediate use. Table 2 lists the wellbore fluids used in the perforating experiments.