An innovative coiled tubing-conveyed treatment combined acid and fluid pulsation to remove wellbore damage.

In its operational area in Egypt's Western Desert, Bapetco has acid-stimulated an exploratory well to restore oil and gas production, which had declined to negligible levels, specifically 1,350 b/d of oil (with 0% water-cut) and 0.5 MMscf/d of gas. After 6 months of production, oil output had stabilized at 600 b/d.
Key to the success was the use of a fluidic oscillator tool (FOT) deployed via coiled tubing (CT) into the production tubing to send pulsing pressure waves through wellbore and formation fluids. The high-frequency pulses can carry energy through well fluids to break down deposits that are blocking the hydrocarbons' path to the well bore. Figure 1 illustrates the physical breakdown of deposits in perforation tunnels.
Background
The exploratory well discussed here was drilled in the Badr El Din field in the Western Desert of Egypt. Original plans were to target a deep formation (lower Cretaceous) in addition to evaluating the upper Cretaceous reservoirs. Openhole logs showed that the deep zone was water-bearing, so it was plugged back, while Sands A and B were found to be oil-bearing and at virgin pressure (5,350 psi). Sand C was gas-bearing, with depleted reservoir pressure (2,500 psi).
The well required a selective completion to isolate Sands A and B oil zones from the Sand C gas zone. A straddle packer with a sliding side door (SSD) allowed separation of the oil-producing zones (to be commingled) from the gas zone that would be produced later through the main production tubing (Figure 2).
The oil zones were perforated first, producing at an initial rate of 450 b/d. The production rate declined rapidly to 200 b/d, and finally ceased to flow within a period of 3 months.
Quality of Sands A and B suggested that even initially the zones were producing considerably less than their potential rates. The formation permeability was estimated to be in the 15 to 30 mD range (minimum value estimates), and based on Nodal analysis simulations (assuming no skin factor), predicted the production rates on the order of 850 b/d to 1,300 b/d.
This analysis led to the belief that the formations were either damaged or the permeabilities were much lower than calculated. Assuming that the well was damaged, the logical source for this damage was a cement plug that was placed across this zone while temporarily abandoning the well. The well was re-entered 2½ months later; the cement plug was drilled out and a liner was set across the open hole. The zones were perforated in a slightly overbalanced condition with 45/8-in. tubing-conveyed perforating (TCP).
Assuming inadequate penetration by the perforating job and workover damage, a hydrofluoric (HF) acidizing job was planned to remove the damage and clean the debris from the perforation tunnels to enhance production from the oil zones.
Meeting the challenges
To enable removal of near-wellbore damage that had halted production from a high-potential exploration well, the operator and service-company personnel followed generally the process below:
They used X-ray diffraction to study drill cuttings for rock mineralogy data (no cores available). Rock was primarily quartz with sodium feldspars, kaolinitic clays, and illitic clays.
The significant carbonate content dictated use of an acid preflush to prepare for a subsequent HF acid stage.
The BHT of 250°F (121°C) and the clay properties yielded a clay-instability rating (CIR) of 86, considered severe. Fortunately, most clays are very stable in organic acids (acetic, formic), even to elevated temperatures. The presence of illite-smectite indicated that clay swelling could be an issue, and therefore, a mixture of organic acids and ammonium chloride was chosen. The modified acid system could (a) remove matrix carbonate and thereby remove drilling damage, (b) dissolve carbonate scale without destroying formation clays, and (c) contact smectite clays without causing swelling.
Acid additives were incorporated to reduce emulsion tendencies, improve water-wetting upon contact with the damage, and prevent corrosion of the tubulars.
Another factor is that fines-migration damage cannot be removed by organic acids. An HF acid system is the answer in such cases. Past experience in other areas of the world with retarded HF acid treatments provided excellent results. A retarded HF acid system is essentially 15% HCl, 1.1% HF, retarded with aluminum chloride. Recent research has also shown this fluid to be particularly compatible with feldspars and illite. The reason for its success is that conventional matrix acidizing with hydrofluoric acid is only effective for removing shallow clay damage 1 or 2 in. from the wellbore, while retarded hydrofluoric acid systems can treat up to 2 to 6 in. from the wellbore, in addition to stabilizing fines migration.
Because there were two formations to be treated (A and B), the overall length of the perforated interval was 85 ft (26 m). It was believed necessary to divert the acid to get a better distribution throughout the zones of interest. A single diverting stage of foamed slug was included to achieve this diversion.
Planners decided to nitrify the entire fluid system to assist in flowing back the well. Nitrogen lifting was also made available to assist in well recovery.
Fluidic oscillation
The acid treatment was delivered downhole via 1¾-in. diameter CT with a fluidic oscillator tool (FOT) fitted to the tubing. Production tubing and completion hardware were left undisturbed.
The FOT system is built around a patented fluidic oscillator. This oscillator creates pulsating pressure waves within the wellbore and formation fluids. These pressure waves help break up near-wellbore damage and restore the permeability by carrying the fluid past the well bore into the formation (Figure 1).
The oscillating pressure waves are not affected by standoff, as with conventional jetting or velocity tools. The kinetic energy of the pressure pulse travels through the wellbore fluid with no appreciable energy loss. When the pressure wave contacts the formation, the energy is dumped, and the process of removing damage is initiated. As the damage is removed and the permeability restored, these pressure waves penetrate deeper into the formation. The pressure waves expand in a spherical fashion from the point of origin providing 360° coverage while moving the tool across the interval. The acoustic streaming induced by the oscillator focuses the treatment and tool energy to the immediate area of the tool.
FOTs work according to the following process:
1. Treatment fluid enters the switch body and is accelerated into the fluidic oscillator.
2. The fluid stream enters the oscillator and preferentially attaches to the outer wall of one of the fluid passageways.
3. The flow continues down the selected passageway to the outlet.
4. As the flow passes a cross channel, a low-pressure area is created that causes the main fluid stream to be interrupted and the flow to switch and attach to the other fluid passageway.
5. The switch begins to oscillate, which causes alternating "bursts" of fluid to be ejected into the well bore.
6. As each "burst" is ejected, it forms a compression wave within the wellbore fluid.
7. Compressive loading occurs when the wave contacts the formation face.
This oscillation enables the pulsation that makes fluidic oscillation so effective in treating formerly inaccessible reservoirs.

Comments