Abstract

Very permeable depleted formations are usually subject to high fluid losses during drilling and completion operations which can lead to formation damage, long clean up times and higher operating costs; these situations are compounded when underreaming and gravel packing are also required.

Lost Circulation Materials (LCM) are often used to limit the fluid losses, but these materials can introduce even greater damage than the permeability impairment caused by excessive fluid loss into the formation.

The selection of the fluid for drilling the producing zone is therefore critical for the success of the completion in order to achieve high performance and reduce the overall operational costs.

A new, non-damaging, water based Drill-In Fluid (DIF) containing a blend of polymers and sized calcium carbonate was identified by an extensive laboratory study to be the most suitable fluid for the drilling and underreaming operations in a well located in the Ionian Sea, southern Italy.

The new DIF was field tested in this subsea well where a depleted and very permeable sand layer had to be completed with an Open Hole Gravel Pack (OHGP).

All phases of the completion operations proceed as planned and no fluid losses were recorded before or alter the gravel pack operations in accordance with laboratory results; clean up was rapid and no matrix stimulation treatment was necessary.

The investment for this DIF was recovered during the clean up phase, since the rig time actually required for this operation was two days less than planned.

The expected productivity was quickly obtained and consequently the same procedures were applied to two other wells in the same area, where the effectiveness of the new DIF has been confirmed.

This paper reviews the results of the laboratory study and the experience gained during the last two years in drilling and completing depleted highly permeable sands; guidelines are established for future applications of DIF in similar reservoirs, particularly where an OHGP has to be installed.

Introduction

The drilling and completion of depleted sands formations has widespread interest. Losses of mud or filtrate can introduce a non-removable damage especially in high permeability reservoirs where the invasion can extend up to and even exceed 10 ft.

The usual choice made by the industry in these cases is to add removable LCM to the drilling fluid in an effort to control losses and reduce the invasion radius. Unfortunately, this procedure very often calls for a matrix stimulation after the completion phase either in cased or open hole situations.

In open hole completions, the best option would be to use a drilling fluid which builds a very thin filter cake capable of withstanding a high overbalance and having a very low Lift Off Pressure (LOP).

Moreover, if an OHGP is the selected completion, the filter cake left at the formation-gravel interface should not damage the gravel pack and the particles created on disaggregation of the filter cake during the clean up of the well should be able to pass through the gravel bed. A thin filter cake will produce a smaller quantity of particles and therefore reduce the risk of damage leading to a higher probability of obtaining a high performance OHGP.

An extensive laboratory study was planned and carried out to evaluate the various DIFs available on the market and to identify the most suitable fluid having the previously described characteristics.

The selected DIF was used to drill and underream a high permeability depleted formation in a well located in the southern offshore area of Italy. The first successful application of this DIF as well as two further applications in a similar reservoir in the same area are reported in this paper.

P. 145^

This content is only available via PDF.
You can access this article if you purchase or spend a download.