Fluidic Oscillation Technique in Conjunction with Acid Stimulation Improves Gas Wells Productivity
Authors: Hegazy Ahmad , Conrad Anthony , Waheed Arsad , Sanad Osama 
Published: February 2016

Generally, the most common reason for low productivity of gas wells in the offshore Mediterranean Sea operations of Egypt is near-wellbore formation damage. Near-wellbore formation damage, referred to as "skin" in petroleum engineering parlance, causes a huge reduction in native formation permeability, thus adversely affecting productivity. Skin damage can occur from one or more of the following reasons: mud and cement damage, mechanical damage from drilling, damage from perforating guns, scale deposition, paraffin and asphaltene depositions, water blocks, gas blocks, acid sludges, emulsions, etc. Dependent upon the type of damage, the stimulation technique will vary; however, the prevalent techniques available to stimulate damaged formations are hydraulic fracturing and matrix acid stimulation.

A new technique called "fluidic oscillation" was proposed to overcome many of the limitations of the previous two techniques. This technique would be especially applicable when there are uncertainties regarding rock mineralogy. In these instances the possibility of further damage exists if incompatible treatment fluids are pumped/" squeezed" into the formations. "Fluidic Oscillation" is primarily a mechanical, nonfluid invasive stimulation technique; (i.e., it is not essential to inject the treatment fluid into the reservoir to break down damage though it does not preclude its use in that manner). The damaged formations are stimulated by subjecting them to alternating bursts of pressure waves generated by pumping fluids through a fluidic oscillator. The treatment fluid can be plain water, diesel, or any other solvent that is desired to pass through the affected zones and be circulated out of the hole. The continuous cycling of these pressure waves causes the skin damage to reach its fatigue failure point. After this, a small amount of acid/solvent blend can be pumped to wash the damaging materials out of the well bore. The fluidic oscillator can be positioned against the zones of interest by using coiled tubing.

The fluidic oscillation technique was used in Egypt on two gas wells (until the writing of this paper) for this operator. In the first well, it is believed that there was heavy formation damage caused while drilling the well. After the stimulation treatment the gas production of the well was increased by 325%. In the second well, the post-stimulation gas production rate was increased by 40%. From IPR curves of the two wells, before and after stimulation, it was found that the absolute open flow (AOF) potential of each well was highly increased after stimulation. Compared to the results obtained from previous treatments, it was found that this technique was successful in stimulating gas wells in Egypt. The main objective of this paper is to evaluate this technique through actual case studies.

Introduction

The main challenge faced after completing two high-rate gas wells was that they were not producing as expected. These wells lie approximately 60 km off the Mediterranean Sea coast in the PetroTemsah field, northwest of the Port Said town in Egypt (Fig. 1). This field produces gas and condensate with condensate/gas ratio (CGR) of 60 bbl/MMscf. The producing formation is the Sidi Salem formation of Serrvallian age (Miocene), with an average porosity of 22%, average water saturation 30%, and an average permeability of 100 md. This formation consists of four main reservoir layers: Lobe-0, 1, 2, 3 (Fig. 2) and is found at a depth of approximately 4,300 m with a bottomhole temperature of 230°F (110°C). The bottomhole pressure ranges between 7,400 to 8,000 psi (521 to 561 kg/cm2). A complete mineralogical breakdown of the rock by weight is not available but essentially the rock is clean sandstone (80% quartz with 3 to 4% calcium carbonate and some 10 to 15% feldspars and clay). The Sidi Salem formation is unconsolidated in nature and without adequate sand control precautions the wells tend to sand-up, ceasing all flow from the reservoirs. The sand control techniques used for the field include openhole gravel packs, expandable sand screens, and cased hole gravel packs. The wells in question have two different types of completion. The first one is an openhole completion with an expandable sand screen set across the zones of interest. The second well has an inside casing gravel pack (ICGP) completion.