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Monday, 29 March 2010

Coiled Tubing Applications for Exploration Drilling at Salym

In 2009, Salym Petroleum Development & Schlumberger assessed the Bazhenov formation - ROGTEC overviews the project and speaks with SPD Well Manager Fred van Nieuwenhuizen about the advantages of coiled tubing.

In Q1 2009 Salym Petroleum Development N.V. (SPD) jointly with Schlumberger Logelco Inc. did an assesment of presense of oil and gas content in Bazhenov formation deposits (JS-0 formation). That is unique geological horizon with unconventional indications of hydrocarbons and reservoirs.

Hydrocarbon reservoirs of Bazhenov formation in most of the cases are represented by shales, enriched with organic content, siliceous deposits and cavernous fractured carbonate rock. One of the most important tasks of Bazhenov exploration is to locate the prospective oil zones using different techniques and strategy. As a part of this effort thee wells were to be drilled in prospective oil zones. One of them characterized by anomalous high temperature >135 degrees centigrade and anticipated formation pressure up to 600 atm.

Possessing necessary expertise, qualified personnel and equipment, Schlumberger Well Services were involved for the project execution and coordination. Initially the well was drilled and cased conventionally by the rig placing section TD into the Upper Bazhenov Member (JS-0). With tubing installed and packer set, the well was handed over to Coiled Tubing for non directional well deepening into underlaying Middle and Lower Bazhenov. Drilling of this section was performed in underbalanced condition in order to in order to appraise the long term unimpaired productivity of the formation. The survey section of the well was tested and then logged by means of wireline. At the final stage, the section was abandoned by setting the cement plug through the coiled tubing in accordance with approved procedures and standards.

The survey section of the well with undefined formation pressure was drilled underbalanced by Coiled Tubing with no danger to people and environment. Collected reservoir characteristics with skin effect eliminated in drilling and during the well test stage. The most complete possible suite of logs was acquired in openhole by wireline. Experience in coiled tubing and ability to adapt this technology for well testing and exploration drilling needs, let Schlumberger to successfully perform this operation.

The following performance indicators were achieved: survey section was successfully drilled with 44mm coiled tubing grade HS-90, 54mm downhole motor and 70mm PDC bit. There were 76 meters of openhole section with clean and stable formation walls penetrating all JS-0 formation. Maximum rate of penetration was 7.2 m / hour. Drilling was done from the top of the formation with azimuth and deviation set by conventional drilling rig. No devices were used for directional control. Dogleg severity was not higher than 2.75 degrees / 30 m. Upon the end of well test and logging, the survey section was abandoned by placing the cement plug through the coiled tubing.

Despite of the fact that oil is produced out of the sandstones in the Lower Cretaceous Cherkashin (AS-11) in this and most of the cases, assessment of presence of oil and gas content in underlying Bazhenov formation deposits (JS-0 formation) is also essential part of license commitment for the company. Peephole underbalanced coiled tubing drilling done by workover department is an example of cost effective solution that delivers on commitment to explore the high-pressure Bazhenov in licensed area without up-scaling the well.

To discuss this project in more depth, ROGTEC talks with SPD Well Manager Fred van Nieuwenhuizen

What advantages did CT have over other possible drilling technology types?

With CT the well could be drilled under balance (otherwise the drilling mud would have damaged the sensitive formation) with water using one pipe size (the coil) which can be "stripped" out of the well under pressure) opposite drilling with tubular joints which have connections of bigger diameter then the pipe which does not have the optionally of circulating while pulling out of hole for the entire length of pipe under full "closed in" condition. (ability to contain the pressure). Actually the standard CT package was designed initially to work underbalanced with surface well control equipment up to 15000 psi. Otherwise the conventional well control stack is designed for 5000psi. Therefore if we want to bring to the rig 15000 psi it will be dramatically expensive for such type exploration projects

Underbalanced drilling has a number of advantages, particularly in our situation using coil tubing drilling. Because there is no mud weight as there would be in conventional drilling, it allows an increased rate of penetration by the drill bit; there is less uncontrolled loss of drilling fluid into the formation strata and there is less potential for the drill tubing to stick to the wall of the well. The potential productivity of the well is minimally affected. In this specific case, the added advantage would be to assess the flow capacity of intersected fractures in the formation whilst drilling, in case a medium (gas/oil/water) would be present in the rock.

It should be mentioned that standard CT package was designed initially to work underbalanced with surface well control equipment up to 15000 psi. Therefore it's possible to find it anywhere worldwide. When the conventional drilling well control stack is designed for 5000 psi surface (in Western Siberia). Therefore if we want to bring the Rig up to 15,000 psi well control equipment it will increase the cost several times.

For the potentially dangerous 600bar formation, how much under-balance were they at on surface?

250 bar under balance in case there would be 600 bar at bottom

How did CT affect the project costs?

Although there is an advantage that the coil tubing unit is a stand-alone unit so no Rig cost, the total set-up with the testing equipment makes it a expensive part of the total well-cost.

In this particular case, that there was insufficient hydrocarbon to justify production, the overall cost of the well was significantly lower than if we had used conventional drilling. The test separator, an integrated element of the set-up, was provided by SPD, as we have this unit in our well services team for general well testing and flow rate calibration on the normal producing assets

Does CT offer additional safety or environmental benefits over standard drilling technologies?

Yes as mentioned the full control over the pressures at all times is a major safety advantage. It's designed to work underbalanced with standard well control surface stack 15000 psi.

Exactly how was danger at surface avoided?

By lowering and pulling the pipe (coil) through a stripper (rubber ring that closes around the pipe) so keeping the fluids and pressure contained. CT is a technique which was designed initially to work underbalanced on any well services operation.

What pressure control stack components (BOPs) were used?

These are special coil tubing BOP's with stripper rubbers and emergency cutting devices, so ability to hold pressure with coil in the well, ability to cut the coil and ability to hold pressure without coil in the well.

How did the penetration rates with CT compare to standard drilling technologies?

Would consider it normal for this depth but very good for the fact that the bit was very small 2.3/4" (70mm) and the well was deep. It could be compared with conventional drilling with a bigger drill-bit (in general the smaller the bit the lower the ROP (rate of penetration) ROP was 2 m/hr (SAV-45 152 mm wellbore)

Will your experience on this field alter your future practices or usage of CT?

Unfortunately was there no presence of oil otherwise SPD would definitely continued with this technology to develop the reservoir. But it should be marked that Shell is promoting a balanced risk-versus reward approach to field exploration efforts, in an overall move to optimize cost and well delivery. To support this, the peephole concept has been promoted globally throughout Shell's global exploration portfolio and been received with great enthusiasm as an additional tool.

Fred van Nieuwenhuizen
Dutch national who obtained a Bsc. in Mechanical Engineering in 1982. He joined Shell the same year and after his initial development in Holland with NAM moved to Oman (PDO), followed by working in Nigeria (NLNG project). He followed that by working in Scotland managing an offshore installation and returned to Holland to become the Well Engineering Course director for the global Shell skillpool. Became the Well Engineering Project Manager for the initial preparation phase of the Kazakhstan Caspian project "Pearls now CMOC" and holds currently the position of Well Engineering manager for SPD (4 Rigs, 7 hoists, heavy transport, fraccing & CTU Ops.)
posted by The Rogtec Team @ 14:35 


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