New Low-Solids Oil-Based Mud Demonstrates Improved Returns as a Perforating Kill Pill
- Ping Jiang (M-I Norge) | Knut Taugbøl (M-I Norge) | Eva Alterås (Norsk Hydro ASA) | Christine Mo (M-I Norge)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2003
- Document Type
- Journal Paper
- 169 - 176
- 2003. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 2.2.2 Perforating, 2 Well Completion, 1.11 Drilling Fluids and Materials, 5.1 Reservoir Characterisation, 5.2 Reservoir Fluid Dynamics, 2.7.1 Completion Fluids, 1.2.3 Rock properties, 2.2.3 Fluid Loss Control, 4.3.4 Scale, 1.14 Casing and Cementing, 1.6 Drilling Operations, 1.8 Formation Damage
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This paper describes the development of a low-solids oil-based fluid and its successful application as a kill pill in long, perforated horizontal wells in the Norwegian North Sea. The authors detail the investigative process that resulted in the development of the heavy, brine-based, internal-phase fluid system.
Simulated perforation formation-damage tests with and without zinc perforating debris were conducted on conventional brine- and oil-based fluid systems. The tests showed that the chemical reaction of the brine-based system to zinc debris significantly increased fluid loss. With the conventional oil-based mud, return flow efficiencies dropped by 10 to 15% when the zinc debris was added.
The newly developed low-solids, oil-based kill-pill system was engineered with a high content of heavy brine for fluid density and minimal solids content. The only solids in the system are calcium carbonate, which is used for bridging, and minimal amounts of clay and lime for rheology control and alkalinity, respectively. In perforation tests, it exhibited more than 80% return flow efficiency.
The first production wells in a field in the Norwegian part of the North Sea showed considerably lower well productivity than expected. Detailed studies were initiated to understand the mechanisms that influenced the productivity of these wells and to identify methods to improve well productivity for the next producers. Strategies for completion, perforation, and completion fluids were given highest priority.
These wells had long horizontal intervals of 1000 to 2000 m. The high reservoir pressure required a high fluid density (~1.65 s.g.) for the overbalance pressure. The horizontal sections were drilled with a 1.65-s.g. oil-based mud. The wells were perforated with a 1.65-s.g. CaCl2/CaBr2 water-based kill pill and low-debris, deep penetrating charges made of a zinc alloy. The perforation was carried out with 12 shots per foot (spf) and 60° phasing. A 1.65- s.g. CaCl2/CaBr2 completion brine was placed above the perforation interval. An in-depth study to determine the reasons for poor productivity revealed possible formation damage from the perforating pill. Poor cleanup efficiency, adverse chemical interactions1 between calcium-based perforating pill and zinc perforation debris, and incompatibility between the kill pill and the formation water containing a high content of HCO3- were identified as possible sources for the damage.
To reduce this risk of formation damage, oil-based completion fluids were investigated. With oil as the continuous phase and low water content as the internal phase, an oil-based system greatly reduces potential interactions with zinc perforation debris. An oil-based system also has good fluid loss control and improved cleanup of the toe of the well because of low viscosity. In addition, an oil-based system is operationally preferable because of high potential time and cost saving. However, conventional, high-density, oil-based systems contain a high percentage of solid weight material, which often tend to plug the perforation channels, seriously jeopardizing the perforation efficiency and ultimately damaging the completion. This created the need for a high-density kill-pill fluid with a low solids content and oil as the continuous phase.
This paper presents the results from extensive laboratory coreflood tests to evaluate the formation-damage potential of water- and oil-based kill pills and the impact of incorporated perforation debris on the damage potential. A number of factors affecting the perforation flow efficiency have been investigated. Based on these studies, an optimum high-density, low-solids, oil-based kill-pill fluid was proposed and optimized to minimize the damage caused by the perforation process. This new fluid system was recently applied as a kill pill in a horizontal-well perforation job in the Norwegian sector of the North Sea. The initial well productivity was ~3 to 4 times higher than in previous comparable wells.
Fluid Systems Investigated
The fluids investigated in the coreflood experiments were of three different types. The first were standard, water-based mud (WBM) kill pills based on heavy brines. Second, a standard oil-based mud (OBM) was tested. Finally, a new type of fluid was developed as an alternative to the conventional OBM based on a heavy emulsion and with very little solids added, called low-solids, oil-based mud (LS OBM). "Low-solids" is defined only as solids required for suspension and bridging, generally less than 180 kg/m3.
Water-Based Kill Pills.
A typical water-based kill pill consists of brine for density and shale inhibition, xanthan polymer for viscosity, starch polymer for fluid-loss control, and sized calcium carbonate for bridging. The WBM fluids tested were 1.26 s.g. NaCl, 1.38 s.g. CaCl2, and 1.55 s.g. CaBr2 WBM. The concentration of calcium carbonate was approximately 170 kg/m3, giving a solid content of approximately 6% by volume.
The conventional OBM consists of a base oil, an emulsifier package, a CaCl2 brine internal phase, barite weighting material for density, and lime and organophilic clay for alkalinity and viscosity, respectively. In addition, fluid-loss-control agents are added. The tested 1.55-s.g. OBM had an oil/water ratio of 75:25. A high barite content (800 kg/m3) was added to give the mud the desired density. Both field and laboratory-mixed muds were tested. The total solids content of these muds was approximately 20% by volume.
Low-Solids Oil-Based Kill Pill.
A new oil-based fluid system to be used as a perforating kill pill was developed as an alternative to conventional OBM. The main purpose was to develop an oil-based system with a minimum amount of solids to eliminate the problems caused by solids in conventional OBM but have the benefits of using an oil-based system at the same time. The desired weight of the fluid is obtained by maximizing a heavy brine content in the internal phase of the system. With a 2.20-s.g. cesium-formate brine and an oil/brine ratio of 40:60, a fluid density of 1.65 s.g. can be obtained without adding any weighting material. For lower-density kill pills, brines with a reduced density can be used. An earlier paper by Nicora et al.2 shows the benefit of using heavy emulsions to form heavy OBM with improved rheological properties. In this paper, the benefit of using a heavy oil-based emulsion with minimum amount of solids as the kill pill is demonstrated. The system is called LS OBM.
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