Speedy, accurate assessment of deepwater reservoirs

September 1, 2013

Standard coring methods often yield incomplete reservoir fluid information. Doug Kinsella, managing director of Corpro, describes a new coring system that provides better information while saving rig time.

Deepwater finds are now playing a crucial role in replacing global reserves, but they also come with new challenges to ensure the reserves are accurately discovered, appraised, and developed safely and efficiently.

Today’s coring operations require superior functionality, performance, and reliability because they are exploring in deeper water, which means higher pressures and higher temperatures.

Drilling in ultra-depwater means higher costs for exploration. Therefore, large reservoirs have to be targeted to have the biggest impact on global reserves. To maximize their efforts, operators are looking to revolutionary technologies to optimize their drilling campaigns.

Accurately assessing the vast reserves available and plotting the best route forward can be a complex process, where the risks, economics, and potential success of an exploration program have to be carefully weighed to fully evaluate the potential performance and deliverability of any exploration program.

Poor reliability and safety risks have often resulted in infrequent use of some traditional coring technologies and methods to analyze pore fluids. Inconsistent analysis has led to reserves estimates that are wide and varied, the results of which are constantly debated.

Deepwater coring

The tool maintains the core at a predetermined pressure and is placed on the rig floor while the next downhole operation begins.Samples that are recovered from deepwater are under extreme stress and pressure. Using current technology, as operators trip out of the hole with a regular conventional static core, the core arrives at surface under atmospheric pressure. During the trip out of the wellbore, there is a release of the tons per square inch of pressure that is on the rock in its natural state. As the core is cut and tripped out of the hole, the formation sample relaxes, allowing gas and liquids to escape. It is the quantity of gas and liquids escaping that become the unknown, resulting in large variations when calculating total reserves.

Rapid depressurizing of the core and gasses forcing their way through the formation can cause fractures that do not exist under reservoir conditions. These fractures can create artificial pathways for gas or liquids to travel that will not be there when the well is produced, exaggerating the well’s potential flow rate. The more pressure on the core in the reservoirs, the greater the problem, which intensifies the situation for deepwater offshore reservoirs. Cores may be pulled to surface with altered permeability equivalent to the core being subjected to a mini frac.

When this occurs and the core arrives at surface, it appears to have many natural pathways for the fluids to migrate out of the reservoir into the wellbore. But when operators go to treat that scenario down hole and emulate those same results, production can be limited. The current belief is these fractures are not actually there. Instead, these fractures are damage that is created during the trip out of the hole. Conventional pressure coring technology can often cause a total loss of pressure, making it impossible to get an accurate picture of the reservoir.

When assessing conventional reservoirs today, operators of offshore conventional wellbores also use a Modular Formation Dynamics Tester (MDT). The MDT latches onto the wellbore in the reservoir sweet spot. The tool creates a draw, or suction, to pull-in fluids and gases from the formation, and brings them to surface at the same pressure as the reservoir. Currently, in a conventional offshore reservoir, firms can only estimate formation reserves with this technology.

Today, operators cut a core and know the volume of that core, but there are empty pore spaces in the core, and it is difficult to determine, with accuracy, if those spaces were filled with hydrocarbons or water. Operators then use the MDT test, but there’s no true way to correlate how much reservoir volume that ssample came from, because it was drawn out of the reservoir near the wellbore wall. From that, operators are forced to estimate how much came out of that known volume of core. They know that the liquids and the gases are from their sample, then they do backwards math in an attempt to understand how much gas and liquid came out.

New technology

Fluids are captured in cylinders and stacked above the core barrel.Operators often define their coring program not by what they want, but rather by what they can afford. They are constantly seeking to secure more core samples at a lower cost, allowing them a better reservoir assessment.

The QuickCapture coring platform allows operators to better understand exactly what they have in their deepwater reservoirs before making critical, fact-based, completion and production decisions, because it reflects the exact amount of hydrocarbons that came out of an exact amount of core volume.

One hundred percent of gas and liquids are captured in storage cylinders by proprietary valves and are stacked above the core barrel. The core is only allowed to depressurize to a certain level during the trip out. The confining pressure is predetermined with the client and that decision is based on a compromise between rig safety and core integrity preservation. Although the core is allowedto depressurize to some degree, all of the subsequent liquids and gases are retained and temperatures and pressures are monitored for the entire trip out of the hole.

Currently, clients are trying to minimize the damage due to the decompression of the core by extending trip time to as much as 48 hours from the bottom of the well bore to surface. While this does not solve the problem of lost hydrocarbons, it is believed that it reduces the mini-frac effect. This can drastically increase the cost of obtaining a core. The tool offers slow depressurization without any associated rig time cost.The tool maintains the core at a predetermined pressure and is placed on the rig floor while the next downhole operation begins. The tool is capable of depressurizing the core over days not hours, while still collecting all gas and liquids migrating from the core. Depressurizing the core becomes an offline procedure, dramatically decreasing the costs, to obtain the added benefit.

This coring technology cuts larger samples (76mm-diameter by 3m-long) and catches fluids that would normally be lost during regular coring operations. These fluids provide data about the reservoir conditions, allowing operators to make quicker decisions and avoid rig downtime.

While the technology is offered for the conventional coring platform, it has proven to be the most cost effective on the QuickCore wireline coring platform, which was specifically designed to enhance the overall speed of the coring process. The tool secures the core in a steel inner tube that holds a thin aluminum sleeve, protecting the core from fluid invasion, while still enabling gases to slowly vent. The aluminum sleeve also serves as a container in which the core can be left in an undisturbed state until the samples are taken to the lab for analysis.

When taking four or five cores in conventional coring, you have to be ready to trip four or five times. Using this system, the assembly is tripped into the hole, cores are cut, never tripping any pipe, just using a wireline, and then tripped back in the hole again. On an average well with four to five cores, this tool can cut the number of trips from ten to two.

In addition to significant time and related cost savings, these tools produce a safer work environment. It is well known the majority of injuries on a rig are from tripping drill pipe—due to the number of activities happening simultaneously and number of personnel involved. Because the tool reduces both the number of wireline trips and drillstring trips required, it minimizes injury potential.

Exploration

More carbonate formations are being drilled offshore in a presalt environment, like those off Brazil and Africa. These formations are naturally fractured, and are difficult to core. For instance, while cutting a 30m core, an operator may cut 8-10m of core before jamming-off. The core then must make a long trip back out of the hole, frustrating operators because of the costs. In this situation, the tool can remove significant time and expense. Instead of taking 24-36 hours to trip out, it takes only 90 minutes on wireline. Using the wireline platform, as soon as a jam occurs, the wireline is dispatched down the hole to pick up the core and bring it out.

A first for the industry

The tool recently completed the first ever 125ft wireline run in a shale gas reservoir, for a major operator in the US. Cutting this core length amounts to a 35% increase in core previously cut over the same recovery time. The operator has used the 90ft assembly on a number of other wells, achieving an average 99.25% recovery across these wells, producing consistent results. Prior to this well, the longest wireline core cut was 60ft with a 3in. diameter. The larger diameter core and 125ft wireline run presents operators with excellent analytical options. The larger diameter core aids in the recovery process while improving core quality. OE

Doug KinsellaDoug Kinsella is managing director of Corpro. He began his career in 1995 with Baker Hughes, where he focused on coring. He joined Corion in 2000, which was ultimately acquired by NOV. In 2009 Kinsella establish Quest Coring, in Edmonton, Alberta, and under his direction as CEO, the company developed QuickCore and QuickCapture.

Quest Coring was acquired by Reservoir Group in May 2012, where Kinsella continues to expand Corpro’s technologies and services. Kinsella earned a mechanical engineering degree from Northern Alberta Institute of Technology.



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