The Qatar Shell gas-to-liquids (GTL) project got off to a good start on 14 February 2005 with the spudding of the first of two appraisal wells in Qatar’s massive North Field. This is the largest gas field in the world, and Shell expects to produce 1.6 billion standard cubic feet of gas per day to use as feedstock in a world-class GTL plant.
This will be built at Ras Laffan and is scheduled to begin production in 2007 using Shell middle distillates synthesis (SMDS) technology. The total cost of the project is estimated at $5bn.
Shell’s programme to drill two appraisal wells in Qatar’s North Field is another important step in Shell’s development of Qatar’s gas reserves and the construction of a second-generation GTL plant, using the shell middle distillates synthesis process at Ras Laffan.
This project heralds the coming of age of GTL technology and a new way of monetising gas apart from using pipelines to deliver gas directly to end customers or converting it into liquefied natural gas (LNG). It also marks the return of Shell to Qatar, a country with great potential for the development of new business.
"It is anticipated that the GTL plant will need a natural gas feedstock of around 1.6 billion standard cubic feet of gas per day. The purpose of the two appraisal wells is to evaluate the two blocks, H4 and H5, which Shell has been awarded in Qatar’s massive North Field. The main objective of drilling the two wells is to confirm the gas composition, which affects the economics of the project and the eventual plant design.
"A second objective is to establish the gas-water contact, which impacts on the volumes in place and allows Shell to assess the porosity and permeability of the formations in which the gas is found, which determines how much gas the well will flow."
The two blocks that have been awarded to Shell are both 144km² in area and 50km offshore from Qatar in a water depth of 30–40m. Production platforms will be built in 2006 or 2007 on the two locations and gas will be produced from 16 development wells and delivered by subsea pipeline to Ras Laffan and, ultimately, into the GTL plant.
Bart Lismont, upstream manager for Qatar Shell GTL, says: "We spudded the first well, NFS1-1, in block H5 on 14 February 2005. The well has been designed to have a total depth of 10,000ft and will be a vertical well.
"Shell will be using ENSCO to drill the two wells and was pleased to get the ENSCO 96 jack-up rig as it was the only rig available with experience of drilling in Qatar waters. The ENSCO 96 was also the best option from the commercial point of view and, most importantly for Shell, the rig has very high HSE [health, safety and environment] standards, with an excellent record over the last four years of no LTIs [lost time injuries].
"The well will be drilled in five sections. We have already drilled the 36in section down to 600ft and laid a 30in conductor pipe. We are currently drilling the 26in section with a roller cone bit. This section will go to a depth of about 2400ft, into which we will lay a 20in casing. The remaining three sections are the 17.5in and 12.25in, with the final 8.5in section going into our target, which is the Khuff gas reservoir at 10,000ft."
Thierry de Meyer, drilling manager at Qatar Shell GTL believes this will not be an easy well to drill, as Shell has no recent experience in Qatar. De Meyer says: "Some of the formations through which we have to drill are fractured and this can cause total losses of drilling mud, which can be both costly and time consuming. These fractures and the possibility of weak hole integrity can also lead to the total loss of a well.
“The top formations are mostly limestone – which is where we can expect the mud losses – then shale, where the drilling will be much harder and we face such problems as getting the complete bottom hole assembly stuck in the well bore.”
One of the main risks associated with drilling wells in the Khuff reservoir is the presence of hydrogen sulphide gas. This is a highly toxic, odourless and colourless gas which can kill at concentrations in the air in excess of 100ppm. De Meyer says: "We expect that the gas we find will have a hydrogensulphide content in the 2–5% range."
The Khuff reservoir is made up of four layers, one on top of the other, and covers a horizontal area of around 80km by 100km. Whereas the reservoir layers are relatively uncomplicated structurally, there is significant heterogeneity in terms of composition and permeability.
The latter determines the quantities of gas the well will flow. Important composition parameters include the condensate gas ratio (CGR) and the inert gas content. The CGR can range from 20–40 barrels of condensate per million standard cubic feet of gas. Inert gases, such as carbon dioxide and nitrogen,can range from 5–15%.
These gases have no commercial value, and while hydrogen sulphide is highly toxic, carbon dioxide and hydrogen sulphide are both highly corrosive. "So in terms of the gas we want to find," says De Meyer, "it should have a low level of inert gases and a high level of valuable condensate, as it is the condensate that is the icing on the cake, being an important element for the commercial success of the project."
Shell aims to bring new technology and added value to Qatar Petroleum’s offshore operations. It hopes to be able to do this by adopting the use of new technologies in its operations, including a novel well test method that is new to the region but which has been successfully tested by Shell in other parts of its global operations.
This new process will cut the well testing time in half, saving some 25 rig days and in excess of $3m per well, without compromising the standards or quality of the data accumulation. In the testing process, the well will be drilled with a fluid that will not damage the integrity of the well bore and Shell will use a liner with external packers and ported collars, through which it will be able to test the well.
According to De Meyer Shell was able to hit the ground running in Qatar by recruiting several key staff from its Soroosh and Nowrooz operations in the Persian Gulf. These people brought with them regional experience and knowledge of the formations, as well as on-the-job experience of Shell’s operating and management systems.
De Meyer adds: "We are also making good use of several of Shell’s global organisations and various centres of excellence around the world. These include Shell Exploration and Production’s well testing team in Houston and its sub-surface team in Rijswijk in the Netherlands.”
Shell expects to take 70 days to drill the first well and then spend 35 days carrying out testing and closing the well. The company will then move the rig to drill the second well.
Analysis of the data from the first well will be evaluated over the following six months and, depending on the results, development of the wells will take place in early 2006.