As a general rule, standard offerings of pressure-while-drilling tools run into problems as temperatures reach 356­°F to 365°F (180°C to 185°C). Tool failure is primarily related to the effect that these temperatures have on electronics and batteries. Without direct measurement, it can be hard to keep downhole circulating pressures within the window between the fracture gradient and the pore pressure gradient. Further, exposure to high temperatures and pressures change fluid properties with time and location in the well as a function of thermal expansion, compression, thermal decomposition of the products, and cuttings disintegration.

While proper fluid design can reduce some of these effects, assuring fluid properties will meet drilling requirements without destabilizing the well bore requires a thorough knowledge of how the fluid will change with time and temperature, and a way of continually estimating the net effect as drilling proceeds.

The high-temperature formation damage testing device allows fluid companies to focus on formation characteristics and fluid interactions. (Images courtesy of M-I Swaco)

M-I Swaco, now a subsidiary of Schlumberger, has developed a combination of laboratory methods for fluids evaluation at up to 572°F (300°C) and a virtual tool that combines this with data taken on site to give drillers a virtual read on current downhole conditions.

Software solutions
M-I Swaco uses Virtual Hydraulics to help mitigate HP/HT risk in a number of drilling environments.As the first software tool designed to look at the effect of temperature and pressure on downhole density and rheology,Virtual Hydraulics is used in the planning stages of an HP/HT project to design fluids that will survive downhole and behave appropriately within a specific range of extreme temperatures and pressures.

“We have to ensure that we can simulate how the fluids will behave under downhole conditions,” said Sanjit Roy, manager of Engineering Technology, M-I Swaco. “Every part of the operation – whether it be drilling, tripping, or running pipe – we have to make sure we know exactly how the fluid behaves.”

With Virtual Hydraulics providing data in the planning stages of a well, M-I Swaco developed PressPro RT to monitor fluid behavior under downhole conditions in real time. “Having a combination of virtual and real-time data provides significant advantages in critical situations where the operating windows are tight and narrow, and allows us to know what the fluidpressures are at every single point in time,” Roy said.

One of the reasons for the success of the programs lies in the fact that mud engineers on the rig can communicate with project engineers in the office through a system of remote operating centers. The main idea behind using these programs is to combine the best available data for the fluid along with the appropriate engineering models. The software is designed to remove the guesswork from determining extreme downhole pressures.

“The inputs that go into the programs are valid and the engineers understand that the software works as a predictive tool for determining downhole conditions,” Roy said.

Real-time information is combined with measurements that can be determined in the laboratory. Extreme HP/HT wells are in excess of 500°F (260°C), and pressures can be in excess of 30,000 psi. Finding conventional test equipment to meet this range can prove difficult. For the purpose of expanding the test envelope, M-I Swaco has assembled a suite of specialized equipment to condition and characterize fluids. The company has a rheometer that is capable of testing at 600°F (316°C) and 40,000 psi. Just conditioning fluids for an extended period of time requires the development of new heat aging cells and ovens to provide for dynamic aging of fluids at temps up to 600°F.

“The purpose for doing this is to actually demonstrate that the fluid will be thermally stable, but getting these measurements are essential to driving prediction programs like Virtual Hydraulics and PressPro RT,” said Mike Freeman, scientific advisor, R&E Fellow with M-I Swaco.

Monitoring of wellbore pressures from remote centers allows safe and efficient drilling.

With the enhanced capacity for determining downhole conditions in the laboratory, this information is combined with measurements engineers make on location. By applying appropriate models to this information, drillers can determine fluid behavior at every point in the well in real time.

“We like to think of our programs as providing virtual sensors for the people out on the rig,” Roy said. In a virtual environment, the software can predict exactly how a fluid is behaving downhole.

A more recent development for these programs is their widespread use of remote operating centers. Real-time monitoring often requires more involvement from the user. It also allows more experts to be involved in the decision-making process. Transmitting data from the rig in real time to a remote center allows a panel of experts to take part and supplement user actions on the rig with more advanced models.

“We like to make decisions fast and send them back to the rig so that the engineers on location do not see just the data, but an actual piece of information or an analysis that they can act on,” Roy said.

Rather than processing data on the fly, remote operating centers allow the computer to do the work, resulting in usable, clear information at the rig site. “Our primary goal is to provide the data fast enough so that the driller can react to the changing conditions in the well and change drilling parameters based on what is happening‘right now,’” Roy added.

Remote centers have been in use for quite some time, mostly for critical HP/HT or deepwater wells, although recent developments in the offshore drilling environment are driving most companies to remotely monitor even standard wells. Programs like Virtual Hydraulics and PressPro RT are making this a simple proposition.

A broadening spectrum
With a number advances in its fluid design strategies, M-I Swaco now has an opportunity to expand its knowledge in regard to rock mechanics properties and mechanical strengths associated with HP/HT reservoirs. Now that the company has merged with Schlumberger, it is developing better insight into these areas.

Traditionally, fluid companies look at rock/fluid interactions from a chemistry point of view, while rock mechanics companies focus on rock responses from a mechanical point of view. Schlumberger, on the other hand, is bringing in-depth knowledge of both fluids and rock mechanics to assist with simulating and modeling rock/fluids interactions under high stresses and HP/HT conditions.

“Recently, some of the rock tests we’ve performed have shown a strong coupling between fluid and rock properties under high earth stresses and HP/HT conditions,” said Quan Guo, manager of Industry Initiatives at M-I Swaco.

Rock behaves very differently under HP/HT with different fluids. The new merger with Schlumberger will give M-I Swaco additional rock mechanics and modeling capabilities for optimizing fluids that specifically address issues in terms of laboratory techniques and modeling approaches.

With this new knowledge, the importance of fluid loss control and managing the invasion of filtrate into the rock has been highlighted. “That’s an immediate change that’s been made to our HP/HT philosophies,” Freeman said.

“We have experience with hundreds of HP/HT wells and a significant investment in equipment and software,” said Lee Conn,vice president of Drilling Solutions. “Schlumberger brings an added understanding of fluid/rock interactions at HP/HT conditions, and we will be using it to further optimize drilling performance on these critical wells.”