An Overview of Air/Gas/Foam Drilling in Brazil
- A.F. Negra˜o (Petrobras S.A.) | A.C.V.M. Lage (Petrobras S.A.) | J.C. Cunha (Petrobras S.A.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1999
- Document Type
- Journal Paper
- 109 - 114
- 1999. Society of Petroleum Engineers
- 1.6.6 Directional Drilling, 5.4.2 Gas Injection Methods, 1.10 Drilling Equipment, 4.2.3 Materials and Corrosion, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.1.2 Separation and Treating, 1.2.5 Drilling vibration management, 2.4.3 Sand/Solids Control, 1.7.1 Underbalanced Drilling, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 5.1.1 Exploration, Development, Structural Geology, 1.8 Formation Damage, 1.7.7 Cuttings Transport, 5.2 Reservoir Fluid Dynamics, 1.11 Drilling Fluids and Materials
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Air, gas, and foam drilling has been used in specific areas in Brazil as an alternative to the traditional drilling process. This work focuses on the benefits and operational improvements obtained with those two-phase flow drilling operations. The comparison between the performance of conventional and two-phase flow drilling has been showing that this technique, when appropriately used, presents some advantages: (1) minimum or no damage to sensitive pay zones, (2) immediate and continuous evaluation of cuttings for hydrocarbon shows, (3) avoidance of lost circulation, and (4) higher penetration rates in hard formations. This work shows recent on-shore two-phase flow drilling field cases in Brazil using both, jointed pipe and coiled tubing techniques. The results obtained in those wells are analyzed and also the surface equipment design, the appropriate technique for connections, as well as the comparison between field data and predictions from simulators are discussed. Future plans for two-phase flow drilling operations in Brazil are also commented. Additionally, potential areas and concrete actions for improving operational feasibility of this technology are identified. The target is a safer and more economical air, gas, or foam drilling operation.
From the middle seventies to the end of the eighties, air and foam technology was applied in Brazil for drilling a series of wells. Some of those applications were mainly focused on the achievement of higher drilling penetration rate. In the Amazon Rain Forest, where the complex logistic is responsible for more than 60% of the total cost of a well, drilling faster represents an expressive impact in the overall expenditures. However, despite all favorable points for use of light weight fluid drilling in this scenario, two wells were drilled with air without accomplishing the expected economical results. Fig. 1 compares the cumulative drilling time of these two air drilling experiences, RUC-18D and RUC-7, with other two typical wells in the area. Although penetration rate was significantly improved, the operational problems associated with the high costs of the additional equipment eliminated the advantages obtained from the improved drilling performance.
In other areas, stable nitrogen foam was used to drill depleted reservoirs. In those areas, the main purpose of the operation was to avoid severe problems of circulation loss, a major setback when using conventional drilling in low pressure zones. A number of wells were drilled in many different old fields in the northeast of Brazil, as Candeias and Carmo´polis.
Also, a foam drilling operation was performed in Garoupa, a mature depleted offshore field in Campos Basin. This particular operational event has shown some difficulties related to offshore two-phase flow drilling, as: (1) lack of space for additional equipment; and (2) logistic limitations. All these problems are being addressed now in a Joint Industry Project currently being conducted in Brazil. This JIP, supported by many oil and service companies plans to drill a pilot well in deep water using light weight fluid in 1999.
These previous and valuable experiences with air and foam drilling showed a great potential for application of the technique in scenarios commonly found in Brazil. However, by that time, the significance of the additional costs and some operational aspects not completely solved like the inadequate performance of surface equipment, mainly the rotating head, hindered the widespread use of this technology.
In 1994, motivated by recent technological improvements in the equipment and also favorable results reported in the literature,1 Petrobras decided to re-evaluate operational and economical benefits of light weight fluid drilling through some field tests. In addition, reservoir pressure in several oil fields had dropped significantly and the large overbalanced condition present in conventional drilling could favor filtrate invasion and the movement of solids, plugging the pore spaces with loss of well productivity.
The first tests were in Carmo´polis Field, an old oil producer field in a heterogeneous conglomerate reservoir, which is supposed to be intensely damaged when drilled conventionally. Sometimes, depending on the well location, usual drilling fluids can also cause total loss of circulation in this field. As a consequence, two directional wells were drilled in Carmo´polis with stable nitrogen foam. The operations were conducted under two different conditions: slightly overbalanced and underbalanced. Despite the additional drilling costs of the nitrogen system, the project reached profitable results. The productivity of the wells grew almost four times when compared to a conventionally drilled well.2
The encouraging results obtained in Carmo´polis Field motivated another application. A vertical well, C-218, was drilled in Candeias Field in the beginning of 1995. In this field, the oil permeates through four layers of fractured rocks in a typical stratigraphic trap. Fractured shales and carbonate rocks with a low pressure gradient of 3.5 lbm/gal compose the main pay zone. This field also presents severe problems of circulation loss. Typically, a vertical well loses around 10,000 bbl (1,600 m3) of water or oil based mud and 160 bbl (25 m3) of drilling cuttings. As a consequence, it takes almost six months for the well to produce back the fluid and to reach expected oil production.
Well C-218 was also a very successful experience, accomplishing the following points: (1) fractured layers were drilled without loss of circulation, (2) the average drilling rate was increased to 9.73 m/h, and (3) the cost to drill the 8 1/2 in. well section was less than the ones normally incurred when using conventional drilling. Figs. 2 and 3 compare the average drilling rate and the costs of well C-218 with others in the area.
Besides accomplishing very satisfactory results, the tests in Carmo´polis and Candeias also improved expertise, increasing confidence on the feasibility of the technology. As a consequence, the first exploratory well was drilled with foam in Parana´ Basin, by the end of 1995.
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