Coiled Tubing Times - oil & gas service, fracturing, coil tubing, well intervention, workover

Journal

Issue 38, November 2011

COILED TUBING TECHNOLOGY - THE INNOVATIVE SOLUTION FOR UNIQUE ASTRAKHAN GAS CONDENSATE FIELD

CHARACTERISTICS OF ASTRAKHAN GAS CONDENSATE FIELD (AGKM)

CT technologies are widely implemented by Schlumberger in Russia for cleanouts of oil and gas wells as well as for production stimulation, logging and also drilling the horizontal wells. CT operations in Astrakhan field are an example of successful experience of CT intervention in highly sour high pressure/high temperature environment.

Astrakhan gas condensate field is extremely harsh in terms of highly corrosive conditions, due to H₂S concentration of up to 26% vol., bottom hole temperature of 110°C and formation pressure as high as 600 bars. All this has a significant corrosive effect on downhole equipment during well production and workover.

The field has been in production since the mid (dle) 80's and there is a tailpipe 100 to 200 meters long installed as a part of completion. The tailpipe (tubing) is overlapping the production zone for 50-100 meters and there-fore highly susceptible to corrosion. That corrosion of tubing metal can result in the coiled tubing becoming stuck during CT operations. During well interventions with CT, the fluid is circulated to the surface with flaring of gas phase after separation. It is not always possible because of towns and villages located close to the field.

CT TECHNOLOGY IMPLEMENTATION FOR WELL WORKOVER IN AGKM

Despite the complications and specifics of AGKM field, CT technology has proven its efficiency. First of all due to significantly lower damage of the near wellbore area comparing to conventional workover, it is not necessary to kill the well with high density drilling mud. At the same time it is required to separate gas and solid phases in treating fluid at the surface, as well as to control contamination of treating fluid with formation fluid. A complex approach that utilizes coiled tubing together with well testing equipment meets all mentioned requirements and also provides the following:

pressure control at the wellhead;
solid phase separation in-stream;
implementation of different types of CT services, such as milling, high-jetting clean out, chemical treatments etc.;
matrix acidizing in one run;
minimization of volumes of treating fluid.

During the well clean out, treating fluid is delivered from surface tanks to the coiled tubing string by high pressure triplex pumps. Flow back from the well goes first through high pressure filters, then through the arrangement of manifolds, gas separator and surge tank. The continuous process is provided by two elements in high pressure filters. Fluid can be pumped through the plugged filtering element to the special tank to remove collected solids.

Wellbore deposits can be removed in different ways, such as with conventional five-orifice wash nozzle, high-jetting rotational tool or with positive displacement downhole motor and bit. During well operations the main issue is CT exposure to H₂S. To minimize it, the production tubing is displaced with a water-methanol solution (40%) before the CT run downhole. The following clean out is performed in slightly overbalanced conditions with 5-10% fluid loss into the formation. Corrosion inhibitor and H₂S chemical scavenger are added to the treating fluid to minimize the effect of corrosion.

It is important to provide the complete return of solids to surface during clean out.

Before the operation, all conditions are modeled with design software to estimate the range of critical parameters. During the treatment, sensor interface hardware is used to monitor and record critical job data and also to estimate remaining tubing life in real-time.

After wellbore clean out, matrix acidizing with hydrochloric acid takes place. CT in that case allows placing acid across the zone of interest and also decreasing volumes and operation time.

DESIGN OF OPERATION WITH COILED TUBING

At the design phase it is required first to define the sequence of techniques that need to be implemented for deposits removal and wellbore clean out. It is effective to run CT with a wash nozzle bottom hole assembly (BHA) to determine the plug depth and also to clean out the unconsolidated plug. Acid can be pumped at that stage to assist in plug removal. In case of hard consolidated plug, which is insoluble in acid, a downhole motor can be used to mill out the plug and clean the wellbore.

At the design stage the following data is critical:

Well data. Properties of wellbore fluids, wellhead pressure, bottomhole temperature, depth, deviation.
Downhole equipment data. Minimal internal diameter (restriction), depth, size, material of production casing and tubing.
Formation data. Pressure, density and viscosity of formation fluid, sour gases concentration, porosity and permeability.
Deposits characteristics. Size and shape of particles, density, solubility, compactness, amount and depth of deposits.
Data of inspection of downhole equipment, production logging data.

With different job parameters, CoilCADE software is used to model CT string loads in the wellbore, maximum weight on bit for milling operations and maximum CT tension load. Distribution of these stresses on the CT string can be calculated at that stage. The optimum regime of clean out is also calculated. With estimated CT loads and also taking into consideration the impact of corrosive environments, the load limits during the operation is calculated as well as the remaining CT life. Load limits and CT life are monitored in real-time during the CT intervention. After every operation an inspection of the coiled tubing string takes place to monitor the damage to the CT string incurred in the wellbore.

CT IMPLEMENTATION EXPERIENCE: WELL # 840

Well history

A workover was performed to change tubular and wellhead equipment. The well was killed and during the workover with conventional rig, high fluid loss/mud loss occurred. To prevent and/or minimize these losses different (LCM) loss circulation materials were pumped such as asbestos particles, cement and bentonite. Total losses during the workover operation were as much as 720 m³.

A packer was installed and pressure tested. During flow back off the well fluid, slugs of mud and gas were observed with tubing pressure drop. Due to unfavorable weather conditions the flowback operation was suspended for 14 days. When flowback resumed two plugs formed which were squeezed down to the formation. The flowback operation was suspended again.

After resuming the flowback, another 4 plugs were removed by pressurizing / bleeding off the pressure to a flare pit. Also chemical agents were pumped to assist in plug removal. After that at the depth 3721 meters another plug was observed and it wasn't removed by technique described above. Using bailing tubes the plug was partially removed only down to 3772 meters. Lab analysis of plug material showed it consisted of clay mud, cement and other LCM materials.

Schlumberger proposed Coiled tubing to clean out that well.

Job design with CoilCADET software

Estimated loads on coiled tubing string during well intervention

Maximum tension load	20 462 lbf
Maximum compression load	-3 363 lbf
Maximum tension stress	2444 bar
Maximum compression stress	2376 bar
Stress, % of yield stress	44.3 %
Load, % of helical buckling load	0.0 %
Maximum weight on bit	2 430 lbf
Maximum tension downhole	19 492 lbf
Depth is reachable	Yes

Figure 2 shows the maximum calculated loads and also working loads versus pressure difference between internal and external pressures.

Figure 4 shows that most of the time, clean out is performed in overbalanced conditions with successful transportation of solids to the surface. Annular velocity during cleanout is not less than 40-50 m/min which is sufficient for solids transportation to the surface in vertical well.

Well workover operation with coiled tubing

Despite the layout with number of units on wellsite, the units are mobile and it is possible to rig down and leave the wellsite in short time. Complete equipment rig up with pressure and function tests takes approximately 16 hours and complete rig down takes approximately 12 hours.

CT operation description

Pumping 20 m³ water-methanol solution into CT and CT annulus to displace the formation fluid from the wellbore and to increase hydrostatic pressure.
RIH with conventional wash nozzle to wireline tag.
Depth 3761 meters. Circulation with treating fluid, pump rate 160-190 liters / min
Place 7 m³ of hydrochloric acid.
RIH with CT down to 3814 m, CT weight decreases at the depth.
Pump 4 m³ of hydrochloric acid.
RIH with CT to 3977 m.
POOH the CT to change wash nozzle to rotating jetting tool - Jet Blaster T.
RIH with circulation of water-methanol solution for matrix acidizing.
Depth 3985 m. Pumping 14 m³ of hydrochloric acid for matrix acidizing.

Jet Blaster tool description

Jet Blaster tool is a rotating tool with high energy jetting effect. It is implemented for deposits removal and consolidated plugs cleanout. It has some advantages, such as high hydraulic energy on the nozzles, 360° coverage of the deposits during cleanout, deposits removal in first run, controllable size of cuttings.

As a result of CT operations the plug was removed with acid squeeze and following circulation with treating fluid. For stimulation of production matrix acidizing of near wellbore area was performed following the removal of filter cake and clean out of production zone with Jet Blaster. Production was restored from 0 to 150 000 m³of gas per day. The plug on that well was removed by acid and a conventional wash nozzle. If there's a consolidated hard plug, the positive displacement downhole motor with mill can be used.

CONCLUSIONS

Implementation of coiled tubing provides additional ways of well clean out and production stimulation in Astrakhan gas condensate field. These methods are chemical as well as mechanical with high-pressure jetting (Jet Blaster tool) and milling (downhole motor). Coiled tubing avoids the need to kill the well prior to the operation and therefore decreasing the damage of near wellbore area and also decreasing the time of well stand by. Matrix acidizing through coiled tubing also has some advantages such as increase of diversion effect by placing the coiled tubing string at the depth of interest and using special diverting system based on viscoelastic self-diverting acid (VDAT). Using Jet Blaster tool increase the efficiency of acidizing by equally distribution of acid on whole production interval.

CT operations have some restrictions due to unfavorable weather conditions and wind direction during the job. To perform operations independently from these conditions it is possible to establish closed loop circulation system which requires additional equipment. This technology is complicated to implement due to highly sour formation fluid and co-operative actions of all parties are required for success.

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