A Novel Technique to Acidize Horizontal Wells with Extended Reach
- H.A. Nasr-El-Din (Saudi Aramco) | N.S. Al-Habib (Saudi Aramco) | M. Jemmali (Schlumberger) | A. Lahmadi (Schlumberger) | M. Samuel (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 26-29 September, Houston, Texas
- Publication Date
- Document Type
- Conference Paper
- 2004. Society of Petroleum Engineers
- 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 3.3.1 Production Logging, 1.8 Formation Damage, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.2.3 Materials and Corrosion, 1.6.1 Drilling Operation Management, 5.8.7 Carbonate Reservoir, 3.2.2 Downhole intervention and remediation (including wireline and coiled tubing), 1.10 Drilling Equipment, 3 Production and Well Operations, 2 Well Completion, 1.11 Drilling Fluids and Materials, 5.7.2 Recovery Factors, 1.6 Drilling Operations, 3.2.4 Acidising
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Drilling horizontal wells with extended reach is intended to maximize reservoir drainage and minimize water production due to water coning. However, an inherent problem with these wells is poor acid distribution during matrix acidizing, especially in reservoirs with high permeability streaks. This paper discusses an innovative approach to treat horizontal wells with extended reach. This new technique comprises mechanical diversion in the wellbore, and chemical diversion in carbonate formations.
Coiled tubing has been used for years to better distribute the acid in vertical and horizontal wells. However, application of coiled tubing in long horizontal wells is a function of wellbore diameter and length. Coiled tubing cannot reach the total depth of the well if there is large washout, or if the length of the openhole is greater than what the CT can reach. The maximum length that CT can reach depends on the length of the reel, diameter of the coil and wellbore geometry. To extend this length, we have used a hydraulic tractor to pull the coiled tubing to the total depth of the well. This will ensure better acid distribution over the wellbore. To enhance acid diversion in the formation, a visco-elastic surfactant-based acid system was employed.
Wells selected are horizontal that were drilled in a carbonate reservoir in the oil fields (both land and offshore) that are present in the eastern part of Saudi Arabia. The total length of the target zone for the well "A" and well "B" is 13,543 and 20,304 ft, respectively. Typical coiled tubing (1 3/4-inch) cannot reach the total depth in these wells (CT lockup length is 10,300 ft for well "A" and 13,200 ft for well "B").
A special hydraulic tractor was used to pull the coiled tubing to the total depth of these two wells. A visco-elastic surfactant based-acid system was utilized to remove formation damage induced by the drilling fluid (water-based mud) and enhance the permeability of the formation in the critical wellbore area. Corrosion inhibitor and other acid additives were carefully selected to maintain the integrity of well tubulars, coiled tubing and the tractor (metallic parts, O-rings and seals of the tractor). Before attempting the stimulation of the extended reach well, a water jetting method was adopted to remove near well bore damage resulting from the drilling mud cake and mud invasion. Production logging tests were conducted after drilling the well, after the water jetting treatment, and after the matrix stimulation. The productivity index of the well decreased after the water jetting treatment. However, the acid matrix treatment delivered through the CT-tractor nearly doubled the productivity index of the treated well.
It is a common practice to drill horizontal and multilateral wells in order to maximize oil recovery in carbonate and sandstone reservoirs. However, when considering these wells for intervention, a major technology gap still exists, especially in extended reach (ER) wells.1 Extended reach wells are defined as those with a measured depth to true vertical depth ratio (MD/TVD) greater than 2.2
Most oil and gas reservoirs are heterogeneous in nature. One would expect significant variations in reservoir permeability in vertical and horizontal wells with long target zones. Significant variations in rock permeability will cause poor fluid distribution during any chemical treatment. In matrix acidizing treatments, poor fluid placement will adversely affect the outcome of acid treatments. Therefore, it is very important to employ methods that improve sweep efficiency during chemical treatments.
Mechanical and chemical means that can enhance fluid placement are available. Mechanical means, including coiled tubing, are used to distribute the fluid across the target zone. It is interesting to note that coiled tubing (CT) has been used in well intervention since the 1960s.3 Field data indicate that CT plays a key role in acid placement, especially for horizontal wells and vertical wells with long target zones.4,5 It is also important to mention that mechanical means improve acid placement around the wellbore area. The effect of mechanical means is minimal once the fluids enter the formation.
Unlike mechanical means, chemical means enhance fluid distribution inside the formation. Several chemical diverters have been used in matrix acid treatments. These include foam,6,7 gelled and in-situ gelled acids.8-10 Nasr-El-Din and coworkers have raised several concerns regarding the use of polymer-based fluids in tight formations, especially in sour environments.11,12
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