The ultra-high wellbore temperatures found in many of the plays around the world today challenge drillers to maintain wellbore stability throughout the drilling and evaluation phases. For the drillbit with its associated bottomhole assembly to be able to reach formations with extreme temperatures, the drilling fluid must be engineered to withstand sustained high-temperature environments.

Wells must be controlled, and it is critical to prevent drilling fluids-related issues during drilling and tripping that could lead to wellbore instability. Furthermore, wellbore conditions must be optimized so that formation evaluation can be performed with the highest quality logging measurements. To address this challenge, M-I SWACO has developed the RHADIANT ultra-high temperature nonaqueous drilling fluid system. The ultra-high temperature drilling fluid system can be applied with various synthetic- and mineral-based fluid chemistries, providing the flexibility to meet specific downhole and environmental demands.

Multiple benefits

While the ultimate goal of the drilling fluid system is to improve well deliverability under extreme hostile environments, the system also delivers added value in several areas. Use in wells offshore Thailand, whose bottomhole temperatures ranged from 204°C to 227°C (400°F to 440°F), showed the following results:

  • Reduced mud losses as a result of stable rheology and enhanced filter cake quality;
  • Enhanced reservoir characterization due to higher quality, thin filter cake;
  • Reduced operating time affecting three activities – tripping time, logging time, and cementing time – all because of unique chemistry that delivers a rheologically stable fluid with reduced gel strengths and corresponding lower swab/surge pressures; and
  • Reduced nonproductive time (NPT) by avoiding stuck logging tools as a result of a slick, thin filter cake.

How does it work?

The ultra-high temperature nonaqueous drilling fluid system eliminates thermal degradation and prevents wellbore control issues due to unscheduled events like barite sag for optimized logging conditions. The system is fully formulated to maintain a stable rheological profile with little maintenance required. It consistently produces an optimized thin filter cake that reduces filtrate contamination of formations and produces a slick wellbore surface. Because of its unique chemistry, the system maintains extreme temperature stability even during prolonged static conditions. Stable rheological properties together with ultra-thin and slick filter cake deposits on wellbore walls make for efficient logging, casing, and cementing operations. These pay off in better designed and executed completion installation and stimulation services. To accommodate environments in which high temperatures are accompanied by corrosives, the drilling fluid system is engineered to resist fluid degradation from CO2 or H2S in addition to using a non-ammonia-forming design.

Three high-temperature products

Accompanying the design of the ultra-high temperature nonaqueous drilling fluid system package are three combinable products that enhance performance.

MUL XT emulsifier. This emulsifier is the principal reason this evolutionary system is able to withstand ultra-high bottomhole temperatures. It also contributes to the system’s very low HP/HT fluid loss values. The emulsifier contains no compounds that can break down at elevated temperatures and release harmful ammonia.

ONETROL HT fluid-loss additive. This component functions as the primary fluid-loss control agent. It is stable in temperatures that exceed 260°C (500°F), with minimal effect on the rheological properties of the drilling fluid.

ECOTROL HT fluid-loss additive. This is the secondary fluid-loss control agent that also retains stability in extreme temperature environments.

The system’s gel strength is carefully balanced to prevent cuttings accumulation and pack-off during connections and also to reduce the surge effect when the pumps are restarted. Unless controlled, progressive gel strength could result in a spike in the breaking circulation pressure at the toolface, potentially causing lost circulation with resulting formation damage, especially in wells drilled with tight margins.

Gulf of Thailand example

One of the areas of sustained ultra-high bottomhole temperatures is offshore Thailand. Out of 178 wells drilled, 33 exceeded 204°C at reservoir depth. The pay sections were drilled using 6 1/ 8 -in. bits. Well deviation reached 60°, and there was possible COand HS. An extensive logging and testing program was planned that would result in static mud conditions for up to four days.

Conventional drilling fluid systems have worked well in the Gulf of Thailand in the past. However, when bottomhole static temperature exceeds 204°C, logging tools have become stuck. Differential pressure sticking mode was suspected. Considerable NPT and lost logging tools encouraged the operator to request a redesign of the drilling fluid package.

A team of M-I SWACO engineers performed tests on several different drilling fluid formulations and determined that the new emulsifier designed for thermal stability in excess of 288°C (550°F) and a filtration control additive comprised of a synthetic polymer coupled with a modified naturally occurring polymer provided the most stable solution. The combination was tested to 274°C (525°F) for stability.

Meanwhile, the team tested the ability of a variety of products to form a thin, tight filter cake at 216°C (425°F) and rejected any that could not deliver this result. In a 16-hr test at 204°C, conventional fluids produced a thick filter cake. The filter cake produced by the new formulation was no more than 1/ 8 -in. thick and was slick and tight after 16 hours.

Lastly, the static shear strength of the new non-ammonia-forming drilling fluid was tested to determine its condition after exposure to ultra-high temperatures. The shear strength correlated well with the pressure used to break circulation after a static period such as making a connection. When circulation was resumed, no pressure spike was observed.

Results show value

After four days of drilling, total depth of the well reached 2,835 m (9,300 ft), and it was time to switch from conventional drilling fluid to ultra-high temperature fluid. When the high-temperature chemicals were added, there was a significant decrease in the yield point and the 6-rpm dial reading.

Even when the drilling fluid contained 20% corrected solids and 4.5% low-gravity solids, the gel structure remained nonprogressive, meaning that breaking circulation would not cause a pressure surge at total depth. Equivalent circulating density spikes when pumps were restarted were negligible. Fluid loss and electrical stability values were improved after the ultra-high temperature chemicals were added. This boded well for the logging operation. The first field trial was performed at 210°C (410°F). It was deemed a success. There were no drilling-related issues while drilling the 6?-in. section. Optimum borehole cleaning was maintained and conformed to prejob hydraulics simulations. Good borehole conditions were experienced during logging and testing operations.

The RHADIANT ultra-high temperature nonaqueous drilling fluid system confirmed expectations and delivered excellent rheological properties, filtration control, and wall cake quality.

Subsequently, additional ultra-high temperature wells were drilled. Each exceeded 204°C bottomhole static temperature.