Fluid loss during the well completion process can be very troublesome and very expensive for an operator if it is not effectively controlled. In some cases, uncontrolled completion fluid loss can potentially cause well control problems that are hazardous to personnel, that threaten the well investment and put the rig and service equipment at risk of loss. Expenses to the operator due to nonhazardous completion fluid loss and resulting control processes include 1) expense of the fluid itself and associated costs such as filtration, 2) cost of rig, materials and service equipment while remedying the fluid loss condition, and 3) cost of deferred or lost reserves if the reservoir is damaged by the fluid losses and the control processes.
Fluid losses can occur in a number of situations:
When the drill-in fluid filter cake remains intact it functions to control openhole completion fluid losses in much the same way that casing and cement control fluid losses for cased-hole completions. The drill-in fluid used during openhole drilling operations is engineered to lay down a thin, low permeability filter cake on the permeable layers drilled through in the pay zone interval. This thin filter cake is composed of specifically sized solids and fluid loss control materials. If the integrity of the filter cake barrier is compromised and significant fluid losses and the resulting control procedures occur, experience shows that completion expenses tend to increase significantly and well productivity often also suffers.
Even using the best completion practices, circulation losses can sometimes occur. Loss zones are classified as "seepage losses" when the loss severity is 1 to 10 bbl/hr, "partial losses" when the loss severity reaches 10 to 500 bbl/hr, and a "complete loss" when loss severity is greater than 500 bbl/hr.1
Effective cleanout and drill-in fluid displacement operations are paramount to attaining high productivity for horizontal openhole completions. Cuttings removal and hole conditioning are required prerequisites for the trouble-free installation of bare screen, expandable screen and gravel pack completions in open holelaterals.
With inadequate circulation to remove cuttings and debris, running screens to depth in the lateral can be hindered or blocked by inferior hole conditions. In addition, when screens are run to bottom in a poorly conditioned lateral, plugging with drilling residue often results in poor productivity.
For gravel pack completions, poor cuttings removal and drill-in fluid displacement can cause annular blockages or screen plugging which block gravel placement. Furthermore, for openhole sand control completions, hole cleaning is the basis for full gravel coverage and screens that are free of plugging debris and ready for unrestricted flow.
New Fluid-Loss Control Technology
One of the newest fluid-loss technologies is Halliburton's LO-GardSM service that helps control fluid loss (leak-off) in perforating/gravel pack completions. It is also effective in horizontal gravel pack applications where fluid loss through the filter cake could cause problems with gravel placement. This service is based on Halliburton's well-proven and proprietary relative permeability modifier (RPM) technology.
LO-GardSM service can also be used in almost any situation where lost circulation occurs including coiled tubing cleanouts, workover operations, post tubing conveyed perforating (TCP) fluid-loss control, and post gravel pack fluid-loss control. The technology is not recommended for use in systems with a high pH and should be mixed only in systems with a neutral or lower pH. The service is compatible with most acids.
The new service is especially suitable for openhole applications because it offers operators a multitude of benefits.
Comparison of Solids Free Fluid Loss Treatments
A comparison of three fluids that can provide fluid-loss control service without using solids was done in the laboratory. The work demonstrates that the three fluids—LO-Gard agent, hydroxyethyl cellulose (HEC) and AquaLinear® service agent—are different and each has different effects on the reservoir's productivity. AquaLinear® agent can give temporary fluid loss control while LO-Gard agent can give extended term fluid loss control.
Figure 1 illustrates fluid volume flow versus time through high permeability sandstone cores. These cores are cut from a sandstone outcrop that typically has permeability in the 2 – 3 darcy range. The flow of fluid through all three cores is initially very high as indicated by the steep slopes of the volume-versus-time curves.
However, a big difference exists between the way HEC and LO-Gard agent affect oil and gas production from a treated well. If the permeability of these treated cores were tested, it would show greatly reduced permeability to oil and gas in the HEC-treated core. Meanwhile, the LO-Gard-treated core would show insignificant change in permeability to oil and gas. Also, the HEC-treated core would require a stimulation treatment to return most of its original permeability. Even then, it would be difficult to effectively contact the entire pay zone with a remedial stimulation treatment to remove this kind of polymer invasion damage to the reservoir in a real well.
In contrast, LO-Gard agent is shown to provide control of fluid losses for brines, but does not require a breaker or remedial stimulation treatment to leave the reservoir oil and gas flow unimpeded.
AquaLinear service treatment affects fluid-flow differently when the treatment fluid is filling the pore space in the core as compared to after it is flushed from the core. While the AquaLinear agent is flowing into the core, the flow rate is greatly reduced by the viscous, shear thinning fluid. Due to its high apparent viscosity characteristic under low shear conditions, the AquaLinear agent's viscosity effectively increases as it flows out into the formation. This makes it capable of controlling fluid losses when placed in the near-wellbore region of the reservoir; however, as the low rate of leakoff pushes the treatment away from the wellbore over time, brine flow would increase to its original high flow-rate. This result is confirmed by tests showing the original core fluid flow rate and permeability are substantially retained after the AquaLinear agent is flushed out of the core.
This shows that with LO-Gard and AquaLinear services, fluid losses can be controlled without the use of treatments that contain particulate solids that plug the reservoir permeability or by the use of polymers that can cause reservoir damage. With LO-Gard service, neither breakers nor remedial stimulation treatments are needed to maintain reservoir deliverability after fluid-loss control treatments. On the other hand, the graphic shows the use of HEC can damage permeability if not properly applied.
Case History No. 1
A South American operator had performed numerous bare screen completions in which the horizontal section required supplemental solids-laden fluid loss control treatments to maintain full circulation returns during the screen installation and subsequent DIF displacement and wellbore clean out operations. These wells were produced with pressure drawdowns ranging from 300 to 400 psi and productivity indices ranging from 10 to 13 boepd/psi drawdown. The best of these wells produced at a rate of 4,600 b/d of total fluid.
A root cause analysis of the below-target performance of these wells indicated an inadequate cleanout and conditioning of the openhole lateral pay intervals. With bottomhole assembly in place for the openhole displacement and cleanup operation, fluid losses into the high permeability reservoir prevented adequate returns rates needed to efficiently displace cuttings debris and drill-in fluid residue from the open hole. Industry-accepted practices indicate that more productive completions are likely to be obtained when circulation velocities in the openhole displacement operation are sufficient to achieve the following:
In a well with a 1,000 foot horizontal section, a plan was developed to optimize the DIF displacement operation and wellbore cleanout before installation of the screens. The main concern with this plan was the potential circulation losses and the possible differential pipe sticking problem.
Halliburton consulted with Repsol-YPF and agreed on the contingency use of LO-Gard service advanced fluid-loss control to reduce losses with a solids-free treatment. Because this technology is new, the customer preferred to test it as a second contingency option, after using a classic carbonate pill. After running the bottomhole cleanout assembly to depth in the lateral and circulating the open hole full with filtered completion brine, fluid losses were 40 bbl/hr.
After placing a 50-barrel carbonate pill in the lateral, fluid losses were reduced to 16 bbl/hr. Since this was not sufficient to enable effective openhole displacement and cleanup, a 35-barrel LO-Gard service solids free treatment (one openhole volume) was pumped. Fluid losses were 2 bbl/hr which allowed the hole to be circulated with filtered brine to a high degree of cleanup as measured by the very low returns fluid solids content of 25 NTU turbidity. After, the screens were installed; the well was circulated again with filtered brine at high rate to obtain 300 ft/min in the annular space without any losses. Finally, the DIF solvent N-Flow™ agent was pumped previous to running the ESP.
The stabilized production rate was 7,000 btf/d with a productivity index of 80 bbl per psi drawdown compared to the best rate of 4,600 b/d with a productivity index of 13 (Figure 2). The ability to produce higher rates at less drawdown allows the reserves to be produced with a lower tendency to cone/crest water from this water-prone field. Based on well test data from this well, the higher production rate alone (additional 810 bopd) provided a 20% incremental daily cash flow of production from the well that used the optimized plan and Halliburton's new fluid-loss control technology.
Case History No. 2
An independent operator in Texas had a horizontal openhole James Lime completion that was producing 300 mscf/d of gas. The well kick-off point was approximately 5,900 ft and the lateral was to 6,756 ft with a BHRP of approximately 200 psi and BHRT of 197°F. During a coiled tubing and nitrogen cleanout operation, the operator could not maintain circulation (fluid was at 5,900 ft).
After conferring with Halliburton, the operator elected to apply a 10 bbl treatment pill of LO-Gard fluid-loss control agent. The well was then jetted dry following a 15 bbl additional treatment pill.
The operator realized an increased production rate of 1.6 mmscf/d on the well following treatment.
1. "Laboratory Study of Lost Circulation Materials for Use in both Oil-Based and Water-Based Drilling Muds," Nayberg, T.M., SPE Drilling Engineering, September 1987.