Unlocking tight gas

The tight reservoir rocks tend to be much older - many were originally deposited during the Palaeozoic era more than 248 million years ago - and typically lack the thick layers of porous and permeable sands that characterise the reservoirs in younger Tertiary basins such as the North Sea and the Gulf of Mexico. Any porosity - spaces between the rock grains - and permeability they once had, has often been greatly reduced as a result of compaction, cementation, recrystallisation and chemical changes during their long and complex burial histories. In conventional reservoirs permeabilities typically range from 0.01 to 0.5 darcy - the darcy is the unit used to measure permeability - but in tight reservoirs permeabilities can be as low as fractions of a millidarcy, or even in the microdarcy range.

As a result many more wells are needed to drain a tight gas field because each well produces a relatively small amount of gas. In addition, the wells need to connect with as much reservoir as possible, so they often have very complex geometries. They may, for example, be drilled on a deviated path to bypass obstacles; or they can be horizontal; or multilateral, where several horizontal wells are drilled in different directions originating from a single vertical well. Some have 'fishbone' geometries, with a central 'spine' and up to 30 small boreholes drilled off at the sides.

Land seismic is a key tool used to optimise well locations and identify the 'sweet spots' in tight gas fields - the areas of improved porosity and permeability. In conventional land seismic surveys the seismic source is generated by pounding on the surface using truck-mounted vibrators, or by setting off buried dynamite charges. The seismic signals that are reflected by geological structures in the earth's surface are detected by thousands of geophone receivers attached to cables laid out in parallel on the ground.

Basic land seismic technology has been around since the 1920s, but thanks to recent work by BP in the tight gas arena the acquisition techniques are being modified to obtain better data, drive costs down and minimise the operating footprint.

The company commands a leading position in the industry in seismic technology for the offshore sector, and is transferring that knowledge to the onshore business with very positive effect. In the offshore arena, innovative seismic acquisition methods developed by BP (Frontiers, April 2007) such as nodes - essentially autonomous recording units - and wide azimuth towed streamer marine surveys, combined with new computer processing techniques, are enhancing the efficiency of appraisal and development of offshore fields. One benefit of these advances is that it is now possible to obtain clear images of reservoirs buried beneath salt - the presence of salt formations distorts seismic signals. Now BP is adapting this leading-edge seismic capability to help in the development of tight gas fields, most of which are located on land, explains Mike Mueller, technology manager for BP North America Gas.

But it's all in a good cause. For example, cableless seismic receiver technology is an onshore adaptation of nodes, combining GPS (global positioning system) with wireless communication technology to eliminate the need for positioning heavy, awkward cables on land - this not only speeds up acquisition, but also allows more sophisticated data transfer. And because the cableless systems can be deployed by men with backpacks, they eliminate much of the ground disturbance associated with conventional land seismic, thus reducing BP's operating footprint.

The first large scale cableless seismic survey was carried out by BP at the end of 2006 in the vast Wamsutter gas field in Wyoming, USA, which, says Mueller, 'has pushed back the frontiers of land seismic technology.'

The Wamsutter field covers an area of around 4000km² - BP is the largest operator in the field, currently with some 1100 wells. The reservoir section is of the order of 600m thick and made up of thousands of tiny gas pay zones, some conventional and already being produced, but many tight, which BP aims to develop.

'To do this, we aim to triple the number of wells, and do it while continuously striving to reduce the environmental impact per well,' explains O'Bryan. 'The new seismic technology is helping us to identify the best well locations. It also helps us to design deviated wells in order to minimise the number required.'

In 2005, BP announced a $2.2 billion investment programme for Wamsutter, involving drilling 2000 new wells over a 15 year period, targeted at tapping into more tight gas (Frontiers, December 2005).

In the Khazzan and Makarem gas fields in Oman, the new seismic technology will be also used to optimise well locations. 'The main goal for the seismic in Oman,' says John Pooler, subsurface and wells director for BP Oman, based in Muscat, 'will be to provide a better picture of the patterns of porosity and permeability.'

In simple terms, fracing technology involves pumping fluids down wells and into the reservoir under pressure in order to create cracks in the rocks to improve permeability. This involves two types of materials: the fluids themselves, and materials known as proppants - very small beads of sand or man-made ceramics - which are used to hold the cracks open.

'Development of fracing technology is being taken further by BP's North America Gas team,' says O'Bryan. 'Stimulation of wells requires a major effort, particularly in tight gas fields where many wells have very complex geometries.'

Developing fluids and proppants having the right physical and chemical properties to stimulate different types of reservoir rock is no easy task. As part of its technology plan, BP is working with a number of university researchers and service companies on new chemistry to create fracing fluids with 'perfect' viscosity and flow behaviour properties, and new proppant materials with 'perfect' strength properties. In this context, 'perfect' will mean right for each job, because reservoir properties vary considerably in different locations.

'The key is to understand which fracing technique works best in which reservoir,' adds Pooler.

'Finding an effective low-power solution to deliquifying these wells will be another step forward in successfully operating tight gas reservoirs,' says O'Bryan. 'To minimise our footprint, we are evaluating pumps that can do the job effectively, along with the ideal motor to run the pumps, powered by solar panels.'

BP believes it is leading the way in introducing, scaling up and implementing the new seismic, well stimulation and production technologies for tight gas, on the global stage. Support between the company's business units in different regions is an important component in this drive forward, exemplified by BP's Tight Gas Symposium, held in May this year, to help BP technologists and engineers from around the world to share experiences on the challenges of implementing different technology applications in a number of tight gas fields.

'In Oman,' notes Pooler, 'we are benefiting greatly from the experience of others in BP in selecting the technologies to apply in the Oman gas fields.'

'This is putting us in touch with fracturing contacts across North America,' reports Cudmore. 'And we've been sharing our seismic expertise in fracture detection with our colleagues in Oman. It's a great example of how we work in a joint way.'

Going from winning technologies in one region to implementing them across BP's business elsewhere is no small undertaking, notes O'Bryan.

'We're working all out on this with contractors and service providers,' he concludes. 'BP is committed to growing our successful technologies to make the production of tight gas easier and more cost effective around the world.'