When dealing with unconventional resources, the technical key drivers which have boosted their exploitation are:
These technologies are not at all new to the oil & gas industry, but they have been considerably improved in the last few years both from the operational and economic side.
Directional drilling is the capability to drill a wellbore deviating its axis from the vertical in a controlled manner.
When producing unconventional reserves, it is common practice to drill a number of horizontal wells through the reservoir to maximize the contact area of the well with the formation.
When producing CBM, the natural fractures of the coal are often sufficient to enable gas flow.
When producing gas or oil shale, hydraulic fracturing techniques have to be used to achieve production.
Hydraulic fracturing is a stimulation technique that aims at substantially increasing the permeability of the rock, and, therefore, the productivity of the wells, fracturing it in controlled way.
This result is generally achieved by injecting fluid into the wellbore at very high flow rate and pressure to induce a network of fractures that propagate for a designed volume (stimulated reservoir volume SRV)into the surrounding reservoir rock formation.
The fractures created in shales by this technique tend to generate more complex and branching fracture networks that expand in multiple direction. As result , the standard, simple and single fracture modeling tools of the past do not provide a valid simulation representation of fracture system. The industry replaced for a valid modeling of the complex fracture network with the introduction of the concept of srv. The srv calculation is based on the effective formation volume defined on the location of the microsismic events that are recorded during the stimulation treatmemt.
A) Shale and shale oil
The shale gas or shale oil exploitation process can be summarized as follows:
Application of horizontal drilling and fracturing for the shale and gas exploitation (Total)
B) Coal Bed Methane (CBM)
CBM extraction process can be summarized as follows:
(following any required treatment) or by reinjection.
In many cases, the low permeability of the coal seam will require the application of ‘stimulation techniques’, and in such cases it is requires :
C) Natural Gas Hydrates
To produce natural gas from hydrates, it is necessary to dissociate them, and three basic methods are available to reach this target, i.e.:
These three techniques are all based on the principle of shifting the hydrate condition from the hydrate stable region to the hydrate unstable region.
1. Depressurization Method
Depressurization method (Research Consortium for Methane)
2. Thermal Stimulation Method
Thermal stimulation method (Oil & Gas Journal)
3. Chemical Injection Method
Gas production from hydrate by the chemical injection method
(Imperial College – UK)
D) Oil Sands
About 20 percent of currently recoverable oil sands reserves lies close enough to the surface to allow open-pit mining.
The bitumen is produced using a strip mining process similar to that for coal mining.
The overburden (primarily soil and vegetation) is removed, and a layer of oil sands is excavated using massive shovels and moved by pipeline or truck to a processing facility where the bitumen is extracted using the hot water technique.
Today, all sites are integrated mine/extraction-upgrading operations which allow extracting the heavy bitumen and upgrading it to a light crude oil called synthetic crude oil (SCO).
Oil sands mining extraction and upgrading process (CBS News website)
In-situ Thermal Processes
About 80 percent of the recoverable oil sands deposits are too deep for surface mining and are produced using advanced drilling techniques combined with thermal processes.
In-situ thermal methods consists of heating the deposit to lower the oil or bitumen viscosity, so allowing it to flow and be pumped to the surface. Heat can be supplied to the formation by injecting steam or hot water, or by starting an “in situ” combustion process.
The thermal processes most widely diffuse today at industrial scale are: Cyclic Steam Stimulation (CSS), Steam Assisted Gravity Drainage (SAGD), and SteamFlooding (SF).
Cyclic Steam Stimulation (CSS)
CSS process (or Huff and Puff) is based on producing steam in once-through steam generators and injecting it down the wellbore into the target formation at a temperature of about 300°C and pressures averaging 11 MPa. This pressure is sufficient to cause parting of the unconsolidated oil sands formation so enhancing the ﬂuid ﬂow.
For each individual well, periods of steaming are followed by periods of soaking and then by periods of production, i.e. CSS is a three-stage process operated in this way:
(1) high-pressure steam is injected through a vertical well for a period of time – (2) the reservoir is shut in to soak – (3) the well is put back on production.
Typical initial cycle times are as follows: (1) injection: 4-6 weeks; (2) soaking: 4-8 weeks;
(3) production: 3-6 months.
Typical recovery rates of CSS process are in the order of approximately 25%-30% of the original heavy/oil/bitumen in place.
Left: CSS in horizontal well (Canadian Natural Resources)
Right: CSS in vertical well (Petroleum Communication Foundation)
Steam Assisted Gravity Drainage (SAGD)
The SAGD is a quite recently developed process originally designed for exploiting the Canadian oil/tar sands. Its implementation requires the drilling of a couple of horizontal wells: an upper steam injector and a lower oil producer. They must run in parallel and be some five meters away.
Steam is injected into the sands through the upper well to create a steam chamber that expands vertically and horizontally. The heat is transferred from the chamber to the bitumen, reducing its viscosity and enabling it to flow – by gravity drainage – into the production well where it is pumped with the condensed water to the surface facilities for processing.
In most cases the expected recovery of SAGD process is in the order of approximately 50% of the original heavy/oil/bitumen in place.
SAGD Process ( Japan petroleum Exploration Co. Ltd)
For the steam-assisted gravity drainage (sagd) well pairs , drilling operations have become very specialized, requiring specially designed drilling rigs and equipment in order to drill wells more and more efficiently without sacrificing wellbore quality and the production performance.
Logging-while-drilling (lwd) sensors help direct high-angle and horizontal drilling and ensure efficient use of expensive rig time.
Measurement-while-drilling (mwd) technology is used to determine wellpath and position in three-dimensional space. Mwd can establish true vertical depth, bottom-hole location, and orientation of directional drilling systems.
Steamflooding is one of the oldest TEOR processes and was substantially developed by analogy with the water or gas injection techniques.
In the SF process high temperature steam is continuously injected through horizontal or vertical wells. As the steam moves through the formation it loses heat to reservoir fluids (usually very viscous oil and water) and rock, and condenses into hot water, which, coupled with the continuous supply of steam behind it, sweeps and drives the oil toward the producer wells.
The oil recovery is enhanced because of the following mechanisms:
From the field data available from a number of SF projects, it appears that this technique is very effective in oil recovery with rates of the order of 50%, and in some favorable cases even of 70%-75%.
Steam flooding (U.S. Department of Energy, Bartlesville, Oklahoma)