GB2526123A

GB2526123A - Producing hydrocarbons from a subsurface formation

Info

Publication number: GB2526123A
Application number: GB1408584.9A
Authority: GB
Inventors: Erling Rytter; Marianne Haugan; Margrete Hã Nes Wasenberg
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2014-05-14
Filing date: 2014-05-14
Publication date: 2015-11-18
Also published as: GB201408584D0

Abstract

A method and apparatus for determining an arrangement of heating wells and production wells in a subsurface formation. A desired ratio of heating wells to production wells is determined, and a unit cell of heating wells and production wells in the desired ratio is determined such that each heating well is located in a defined position relative to a production well, the unit cell being defined in a plane perpendicular to a main axis of the wells. A hot region area is determined for each unit cell, the hot region area comprising a region in which three heating wells are closer to the hot region than a production well. In the event that the hot region area is higher than a desired hot region area, the position of at least one of the heating wells and production wells relative to the other wells in the unit cell is altered and the steps repeated.

Description

Producing Hydrocarbons from a Subsurface Formation

TECHNICAL FIELD

The invention relates to the field of producing hydrocarbons from a subsurface formation when using an arrangement of in-situ heating wells in the subsurface formation.

BACKGROUND

In order to improve the production rate and yield of hydrocarbons located in a subsurface formation, it is known to stimulate production. There are several techniques used for stimulation, which may be used alone or in combination with other techniques, depending on the type of hydrocarbon and the nature of the subsurface formation.

Examples of stimulation techniques include heating, fracking, acid stimulation, Steam Assisted Gravity Drainage (SAGD) and so on. For example, fracking may be performed to induce cracks in the formation from which fluid hydrocarbon can flow.

Acid stimulation may be performed to dissolve part of the rock in the formation, such as carbonates. For some subsurface formations, there might be a need for both fracking and heating. Hydrocarbons that can benefit from heat treatment are typically bitumen, heavy oil, extra heavy oil, tight oil, kerogen and coal. Oils are often classified by their API gravity, and gravity below 22.3 is regarded as heavy and below 10.00 API as extra heavy with bitumen being typically around 8° API. These heavy oil sources may be termed "unconventional". In order to increase production of hydrocarbons from unconventional sources requires development of highly efficient enhancement and stimulation methods of the reservoir comprising heating as one part of the operation.

Note also that other hydrocarbons such as shale gas and oil and coal bed methane may benefit from stimulation.

Shale reservoirs are hydrocarbon reservoirs formed in a shale formation, often denoted as shale oil, shale gas or oil shale. It can be difficult to extract the hydrocarbons from shale reservoirs because the shale formation is of low porosity and low permeability.

This means that when a well is drilled into the formation, only those fluid hydrocarbons in proximity to the well are produced, as the other hydrocarbons further away from the well have no easy path to the well through the relatively impermeable rock formation. In order to improve hydrocarbon recovery from shale reservoirs, the shale around the well is often hydraulically fractured. This involves propagating fractures through the shale formation using a pressurized fluid. These fractures create conduits in the impermeable shale formation. Hydrocarbon fluids can then migrate through the conduits toward the production well. In this way, recovery of hydrocarbons from the reservoir is improved because hydrocarbons that would not previously be able to reach the well now have a path to the well and can be produced.

The term "oil shale" refers to a sedimentary rock interspersed with an organic mixture of complex chemical compounds collectively referred to as "kerogen". The oil shale consists of laminated sedimentary rock containing mainly clay minerals, quartz, calcite, dolomite, and sometimes iron compounds. Oil shales can vary in their mineral and chemical composition. When the oil shale is heated to above 260-370 CC, destructive distillation of the kerogen, referred to as pyrolysis, occurs to produce products in the form of oil, gas, and residual carbon. The hydrocarbon products resulting from the destructive distillation of the kerogen have uses which are similar to other petroleum products. Indeed, oil shale is considered to have potential to become one of the primary sources for producing liquid fuels and natural gas, to supplement and augment those fuels currently produced from other petroleum sources.

Known in situ processes for recovering hydrocarbon products from oil shale resources describe treating the oil shale in the ground in order to recover the hydrocarbon products. These processes involve the circulation or injection of heat and/or solvents within a subsurface oil shale. Heating methods include hot gas injection, e.g. flue gas or methane or superheated steam, hot liquid injection, electric resistive heating, dielectric heating, microwave heating, or oxidant injection to support in situ combustion.

Permeability enhancing methods are sometimes utilized including; rubblization, hydraulic fracturing, explosive fracturing, heat fracturing, steam fracturing, and/or the provision of multiple wellbores.

Heating fluids can be one of several types. Often a molten salt is used, such as a nitrate or carbonate salt, or a mixture of such salts. An example of a heating fluid is a mixture of 60% NaNO3 and 40% KNO3 with a melting point of 220CC. This mixture can be heated to 45065OcC, preferably between 55060O0C, before being piped into to the reservoir. The return temperature at the surface for reheating is typically around 250- 500CC, preferably 300-450°C. Other classes of suitable salts include carbonates, halides or other well-known anions. The counterion (cation) should be environmentally benign, essentially in the form of alkali, alkaline earth elements or sink. A further option is imidazolium based counterion it a low melting temperature is required. In general, a large size counterion gives a low melting point due to reduced coulomb interactions.

The use of molten salts as a heat transfer fluid for heating a subsurface formation has been described in US 7,832,484, which also includes several examples of such salts.

Note that it is also possible, with due consideration of cracking effects, to use a hydrocarbon as heating medium. The hydrocarbon can be in a gaseous or liquid form.

The heating fluid is returned to the surface. In the surface facilities, the heating fluid is reheated after having been cooled down in the reservoir formation. Furthermore, it may be necessary to remove unwanted impurities in the heating fluid that have been picked up in the reservoir. Certain aspects of U-shaped wellbores containing heating fluid in a closed loop heating system have been described in WO 2006/116096.

In order to heat the subsurlace formation, a number of heating wells are provided along with a number of production wells for the produced hydrocarbons to be transported to the surface. Several arrangements of these wells have been described, for examples in WO 2009/052044. Known arrangements of heating wells and production wells are a hexagonal, or near hexagonal, arrangement of heating wells around a production well.

A close packing of heating wells in the formation section to be heated gives each set of three neighbouring heating wells a substantially triangular shape when viewed in cross section along the main axis of the heating wells. Production wells are located within each triangle of heating wells. Depending on the distance from the production well, the distance between the heating wells may vary, e.g. being denser further away from the production well. The reverse is also feasible, that is with closer spacing in the inner sections. A typical ratio of heating wells to production wells is 7:1.

An alternative arrangement of heating wells has been described in WO 2009/142803.

In this arrangement, one production well is surrounded by two layers of heating wells, each in a pentagonal pattern that is staggered relative to each other. This gives a ratio of 8 heating wells per production well, but this document does not describe an arrangement in a reservoir which extends over many production wells.

Drilling and completion of wells are major cost factors in production of hydrocarbons from subterranean formations, particularly when the production relies on heating to the subsurface formation. It takes a long time to heat the subsurface formation, typically many months or several years, depending on the desired final temperature, the type of heat source available and the characteristics of the subsurface formation. It is therefore necessary to optimize the well pattern, both of heating wells and production wells, and to extend the well pattern to the entire cross-section of the hydrocarbon bearing regions of the subsurface formation. Of course, at the periphery of the cross-section, the pattern may be truncated and may have to be modified, among others to minimize heat loss to the surrounding rock.

SUMMARY

An object is to address the problem of inefficient heat distribution and therefore excessive energy consumption.

It has been realized that overheating certain parts of the subsurface formation should be avoided. The hydrocarbons generated (or stimulated) by the heat from a heating well should, as much as possible, be able to migrate to a production well without passing close to another heating well. It is therefore necessary to minimize regions of the reservoir that are surrounded with heating wells and have no nearby production well. Apart from unnecessary heat input, giving rise to higher costs and enhanced greenhouse gas emissions, such hot regions can give rise to secondary cracking as well as gasification that reduces the oil production yield from the reservoir.

According to a first aspect, there is provided a method of determining an arrangement of heating wells and production wells in a subsurface formation. The method comprises: (i) determining a desired ratio of heating wells to production wells; (H) determining a unit cell of heating wells and production wells in the desired ratio such that each heating well is located in a defined position relative to a production well, the unit cell being defined in a plane perpendicular to a main axis of the wells; (Hi) determining for the unit cell a hot region area, the hot region area comprising a region in which three heating wells are closer to the hot region than a production well; (iv) in the event that the hot region area is higher than a desired hot region area, altering the position of at least one of the heating wells and production wells relative to the other heating wells and production wells in the unit cell, and repeating steps (H) to (iv).

As an option, the method further comprises determining a pathway between each heating well and an adjacent production well, and if the pathway traverses a hot region, repeating steps (H) to (iv).

The position of at least one of the heating wells and production wells relative to the other heating wells and production wells in the unit cell may cause a new unit cell to be defined.

As an optional, in the event that a group of heating and production wells comprises a substantially regular polygon having at least four corners, with a production well occupying one of the corners, determining that the group of heating and production wells does not contain a hot region area.

According to a second aspect, there is provided a computer device arranged to determine an arrangement of heating wells and production wells in a subsurface formation. The computer device comprises a user input device for receiving a value of a desired ratio of heating wells to production wells, and a processor for determining a unit cell of heating wells and production wells in the desired ratio such that each heating well is located in a defined position relative to a production well, the unit cell being defined in a plane perpendicular to a main axis of the wells. The processor is further arranged to (i) determine for the unit cell a hot region area, the hot region area comprising a region in which three heating wells are closer to the hot region than a production well. The processor is further arranged to (H) determine if the hot region area is higher than a desired hot region area, and, if so, altering the position of at least one of the heating wells and production wells relative to the other heating wells and production wells in the unit cell, and repeating determinations (i) to (H).

As an option, the processor is further arranged to determine a pathway between each heating well and an adjacent production well, and if the pathway traverses a hot region, repeat steps (i) and (H).

The processor is optionally arranged to alter the position of at least one of the heating wells and production wells relative to the other heating wells and production wells such that a new unit cell is defined.

As an option, the processor is further arranged to determine that if a group of heating and production wells comprises a substantially regular polygon having at least four corners, with a production well occupying one of the corners, then the group of heating and production wells is not considered to contain a hot region area.

As an option, the processor is arranged to at least in part, use a reservoir model to determine the unit cell, allowing the most suitable unit cell for the type of formation to be selected.

According to a third aspect, there is provided a computer program comprising computer readable code which, when run on a computer device, causes the computer device to perform the steps of: (i) determining a desired ratio of heating wells to production wells; (H) determining a unit cell of heating wells and production wells in the desired ratio such that each heating well is located in a defined position relative to a production well, the unit cell being defined in a plane perpendicular to a main axis of the wells; (Hi) determining for the unit cell a hot region area, the hot region area comprising a region in which three heating wells are closer to the hot region than a production well; (iv) in the event that the hot region area is higher than a desired hot region area, altering the position of at least one of the heating wells and production wells relative to the other heating wells and production wells in the unit cell, and repeating steps (H) to (iv).

According to a fourth aspect, there is provided a computer program product comprising a non-transitory computer readable medium and the computer program described above in the third aspect, wherein the computer program is stored on the computer readable medium.

According to a fifth aspect, there is provided a system for producing hydrocarbons from a subsurface formation. The system comprises a plurality of production wells, at least part of each production well located in a portion of the subsurface formation. A plurality of heating wells is also provided, at least part of each heating well located in a portion of the subsurface formation. The production wells and heating wells are disposed to form a pattern in the subsurface formation such that hot regions form less than 65% of the area of the portion of the subsurface formation, each hot region comprising a region in which three heating wells are closer to the hot region than a production well.

As an option, in the event that a group of heating and production wells comprises a substantially regular polygon having at least four corners, with a production well occupying one of the corners, then that group of heating and production wells is not considered to contain a hot region area.

As an option, hot regions form less than any of 50% of the area of the portion of the subsurface formation, 35% of the area of the portion of the subsurface formation, and 25% of the area of the portion of the subsurface formation.

The pattern of heating wells and production wells in a cross section perpendicular to a main axis of the wells optionally forms a repeatable unit cell within the subsurface formation. In this case, the unit cell optionally takes the shape of any of a triangle, a rectangle, a pentagon, a hexagon, a rhombus and a parallelogram.

As an option, a largest dimension of the unit cell is any of less than fifty times an average distance between adjacent heating wells, less than twenty times an average distance between adjacent heating wells, and less than ten times an average distance between adjacent heating wells.

As an option, a sum of distances from a production well in the unit cell to all adjacent heating wells closer than a closest adjacent production well is less than a peripheral distance of the unit cell.

The pattern of heating wells and productions wells is optionally any of close packing of wells, cubic packing of wells, interlinked pentagons of wells, pentagons of wells linked by octahedrons, and truncated hexagons of wells. As a further option, a production well is ocated at an interstitial site within a pattern of heating wells. As a still further option, the system further comprises a second pattern at a different region of the subsurface formation.

As an option, a distance between neighbouring wells is substantially uniform throughout the pattern.

A ratio of heating wells to production wells is optionally between 2 and 7.

The heating wells are optionally arranged to heat the surrounding formation to a temperature sufficient to crack and/or pyrolize kerogen. Such a temperature range is optionally in the range of 100°C to 450°C.

Any suitable type of heating wells or combinations of types of heating wells may be used. Examples include hot fluid injection, closed loop hot fluid, electrical heating, gas heating, and induction heating. Examples of heating fluids include molten salt and an ionic liquid.

According to a sixth aspect, there is provided a method of producing hydrocarbons from a subsurface formation. The method comprises providing a plurality of production wells, at least part of each production well located in a portion of the subsurface formation, and providing a plurality of heating wells, at least part of each heating well located in a portion of the subsurface formation. The production wells and heating wells are disposed to form a pattern in the subsurface formation such that hot regions form less than 65% of the area of the portion of the subsurface formation, each hot region comprising a point at which three heating wells are closer to the hot region than a production well. The method further comprises heating the subsurface formation using the heating wells and producing hydrocarbons using the production wells.

As an option, the method comprises disposing the production wells and heating wells such that hot regions form less than any of 50% of the area of the portion of the subsurface formation, 35% of the area of the portion of the subsurface formation and 25% of the area of the portion of the subsurface formation.

The production wells and heating wells are optionally disposed such that the pattern of heating wells and productions wells is substantially any of close packing of wells, cubic packing of wells, interlinked pentagons of wells, pentagons of wells linked by octahedrons and truncated hexagons of wells. A a production well is optionally disposed at an interstitial site within a pattern of heating wells. A second pattern of wells is optionally disposed at a different region of the subsurface formation.

As an option, the method comprises using the heating wells to heat the surrounding formation to a temperature sufficient to crack and/or pyrolize kerogen (for example in a range of 100°C to 450°C).

A group of heating and production wels in the unit cell comprising a substantially regular polygon having at least four corners, with a production well occupying one of the corners of the polygon optionally does not contain a hot region area.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 illustrates schematically a first exemplary arrangement of heating wells and production wells; Figure 2 illustrates schematically a second exemplary arrangement of heating wells and production wells; Figure 3 illustrates schematically a third exemplary arrangement of heating wells and production wells; Figure 4 illustrates schematically a fourth exemplary arrangement of heatng wells and production wells; Figure 5 illustrates schematically a fifth exemplary arrangement of heating wells and production wells; Figure 6 illustrates schematically a sixth exemplary arrangement of heating wells and production wells; Figure 7 illustrates schematically a seventh exemplary arrangement of heating wells and production wells; Figure 8 illustrates schematically an eighth exemplary arrangement of heating wells and production wells; Figure 9 illustrates schematically a ninth exemplary arrangement of heating wells and production wells; Figure 10 is a table listing properties of heating well and production well arrangements with 3 heating wells per production well; Figure 11 illustrates schematically a tenth exemplary arrangement of heating wells and production wells; Figure 12 illustrates schematically an eleventh exemplary arrangement of heating wells and production wells; Figure 13 illustrates schematically a twelfth exemplary arrangement of heating wells and production wells; Figure 14 illustrates schematically a thirteenth exemplary arrangement of heating wells and production wells; Figure 15 illustrates schematically a fourteenth exemplary arrangement of heating wells and production wells; Figure 16 illustrates schematically a fifteenth exemplary arrangement of heating wells and production wells; Figure 17 illustrates schematically a sixteenth exemplary arrangement of heating wells and production wells; Figure 18 illustrates schematically a seventeenth exemplary arrangement of heating wells and production wells; Figure 19 illustrates schematically an eighteenth exemplary arrangement of heating wells and production wells; Figure 20 illustrates schematically a ftneteenth exemplary arrangement of heating wells and production wells; Figure 21 illustrates schematically a twentieth exemplary arrangement of heating wells and production wells; Figure 22 is a table showing features of additional heating well and production well arrangements as illustrated in Figures 11 to 21; Figure 23 is a flow diagram showing exemplary steps for arranging heating wells and production wells; Figure 24 is a flow diagram showing exemplary steps for determining an arrangement of heating wells and production wells; and Figure 25 illustrates schematically in a block diagram an exemplary computer device.

DETAILED DESCIPTION

It is proposed to arrange the production and heating wells in such a pattern that the number of heating wells per production well is reduced compared to the typical ratios between number of heaters to producer of 7:1 or higher. The occurrence of hot regions (sometimes termed "hot-spots') is minimized or even in some cases eliminated. The pattern of production wells and heating wells can be extended and repeated over an entire cross section of the reservoir by a simple geometric repetition.

One way to describe an arrangement of heating wells and productions wells in a two dimensional array (when viewed in cross section perpendicular to a main axis of the wells) is to use the notation of unit cells. A unit cell, as applied here in the description of heater and producer arrangements, is a geometrical figure that can be repeated in directions in a two-dimensional space to build up an entire space without gaps between the unit cells and without overlapping of portions of adjacent unit cells. It is convenient, if possible, that such a unit cell has a regular shape that can be repeated within a Cartesian coordinate system, although other systems also are possible. It is convenient to define a unit cell in the form of a paralelogram. A parallelogram has four sides with two pairs of parallel sides. The opposite sides are of equal length and the opposite angles are equal. Special cases are a rectangle where all the angles are 900, a rhombus where the sides are equal in length, and a square where both the lengths and angles (9Q°) are equal. Additional features of a parallelogram is that adjacent angles are supplementary and add up to 180°, it has a rotational symmetry of second order, and that the sum of the squares of the four sides equals the sum of the squares of the two diagonals. Furthermore, the sum of the distances from an interior point in the parallelogram to the sides is constant irrespective of the location of the point.

A hot region is defined herein as region of points in the unit cell where there are at least three heating wells closer to those points than a production well. In the examples described in WO 2009/052044, the hot regions take up more than 80% of the area of the subsurface formation in which the production wells and heating wells are located.

In WO 2009/1 42803 hot regions take up more than 65% of the area of the subsurface formation in which the production wells and heating wells are located. In the arrangements described herein the hot regions take up less than 65% of the area of the subsurface formation in which the production wells and heating wells are located.

The term "area" is used herein because the structure of heating wells and production wells is described in a two-dimensional plane perpendicular to a main axis of the wells.

It will be appreciated that, in practice, a hot region occupies a volume of space.

However, for clarity, the term hot region area is used to define a hot region in terms of two-dimensional space.

In most of the arrangements described below, instead of placing production wells between the heating wells, as is known, the heating wells and producton wells are collectively spread out in a regular geometric pattern (e.g. a production well may occupy one corner of a regular polyhedron). This principle results in an equal closest (next neighbor) distance between the wells. In the geometrical pattern, the production wells are spread out in a way that minimizes overheated regions. Efficient arrangements of heaters and producers that minimize hot regions can be deduced automatically using a mathematical subroutine of a computer program, e.g. for a given ratio of heaters to producers.

The examples contain many options of optimized well patterns sorted after the number of heating wells per production well. If there are only two heating wells per production well it is trivial to avoid hot regions. However, this low ratio usually provides inadequate or non-homogeneous heating throughout the subsurface reservoir, and so it is desirable to increase the ratio above 2. However, as this ratio of heating wells to production wells increases, it becomes increasingly difficult to avoid or minimize hot regions.

The geometric patterns of heating wells and production wells described herein reduce the risk of overheating in local areas. Overheating may cause unwanted reactions, in addition to using excess energy to heat the subsurface formation. Furthermore, any produced oil and/or gas that passes through an overheated region may undergo undesirable secondary reactions such as polymerization or coking.

It will be appreciated that the production wells and heating wells may be disposed or deviated substantially horizontally, substantially vertically, or at any suitable angle depending on the structure of the subsurface formation. In the figures, a production well is in one repetitive position of the geometric pattern rather than simply being located inside a heating well pattern. This principle allows for the majority of the produced hydrocarbon in the subsurface formation to move to a production well with minimal influence from hot regions.

It will be appreciated that the subsurface reservoir is not homogeneous. There may be faults, uneven shape of the cross-section, varying porosity and composition of the formation etc. There may be layers within the subsurface formation having different composition, hydrocarbon content and maturity. Fracks may also influence the flow pattern of hydrocarbons towards a production well. Fracks may be deliberately induced or be a product of the heating progresses. For these reasons, it may be advantageous to modify the original designed geometrical pattern of the wells within the same subsurface reservoir. Such modifications may involve truncation, distortion or compacting of certain sections, and may also involve adding additional wells or removing wells if required. It is also possible to move from one geometric pattern of heating wells/production wells in a first section of the subsurface formation to another geometric pattern of heating wells/production wells in a second section of the subsurface formation. It is to be understood that such modifications still is within the scope of the appended claims.

Figures 1 to 9 show exemplary arrangements of heating wells and production wells shown n cross section in a plane perpendicular to a main axis of the heating wells and production wells. Their characteristics are summarized in Table 1. The arrangements in Figures 1 to 9 have a ratio of three heating wells per production well. Heating wells are shown with light shading and production wells are shown with dark shading. Unit cells are shown with a dashed outline and hot regions with solid lines and/or the letter H. The shortest pathway for hydrocarbons to take from a heating well to a production well is indicated by an arrow.

Figure 1 is characterized by close packing of heating wells and production wells and a rectangular unit cell comprising a total of four wells, i.e. the smallest unit cell possible with three heating wells and one production well. The hot region is only 25 % of the total area in the unit cell, far below any known configuration when there are at least three heating wells per production well. There are six heating wells surrounding one production well and no pathway from a heating well to a closest production well goes through or even touches a hot region. This configuration is advantageous in minimizing hot regions and reducing the probability of produced oil and gas being exposed to excess temperatures. It is surprising that these advantages can be obtained through a comparatively simple arrangement ol the wells.

The well configuration in Figure 2 has many of the beneficial characteristics of Figure 1, but with a somewhat larger hot region and a significantly larger unit cell. In this more open structure there are only as an average 0.5 hot regions per production well whereas the ratio in Figure 1 is 1:1.

In Figure 3, the heating wells are in a close packing arrangement, but the production wells are disposed in interstitial locations between the heating wells. This leads to very short distances between heating wells and production wells, all of the shortest distances being less than 60% of the distances between the heating wells. However, there are only three close heating wells per production well and the fraction of hot region s greater than 65%. The unit cell is a small rhombus or can alternatively be described by a twice as large rectangle. This example illustrates that placing the production wells separate from the overall pattern of wells in interstitial positions does not give a pattern in which hot regions form less than 65% of the area of the subsurface formation.

Figure 4 shows a cubic packing of wells giving a rhombohedral unit cell. In this case it is convenient to describe the structure by a twice as large rectangular cell.

Surprisngly, there are no hot regions in this configuration. The region confined by the heating wells surrounding a production well takes up 50% of the total area, whereas a production well is part of all the squares that define the structure, thus eliminating potential hot regions.

Both pentagons and octagons make up the configuration in Figure 5, and neither of these are hot region areas as defined above. The pentagons contain at least one production well, whereas all octagons contain two production wells. Consequently there are no hot regions. In this example, most heating wells are 3 m from the nearest production well in this arrangement, 3 m being the distance between close wells.

However, other distances may be used and there are advantages in using larger distances. The structure can be described by bands of pentagons alternating in direction up and down and arranging these bands to give octagons between them. It can be seen that by following the wells constituting the upper, or lower, periphery of the bands, each fourth well is a production well.

Figure 6 is a variation of the arrangement shown in Figure 5. There are bands of alternating pentagons, but the bands are joined together giving truncated hexagons.

This means that some wells will be common to two bands of pentagons. Due to the truncated nature of the hexagons, it can be argued that there will be hot regions in the quarter of the hexagons that contain only one production well, as indicated on the figure. However, the hot regions constitute only around 9% of the total.

Both Figures 7 and B are composed of hexagonal arrays of wells, the difference being that in the latter structure it has been indicated that two different types of heating wells can be applied. This may be, for example, electric heaters and heating wells using heating fluids. Halt of the hexagons contain two production wells, whereas the other half contains one. It can be seen that the three heating wells of the hexagon furthest from the production well are close to production wells in adjacent hexagons.

Figure 9 is a variation of Figure 7 where the triangular configuration of heating wells surrounding a production well has been truncated. Although the structure appears rather different, the position of the production wells is unchanged and the structure can be described by the same unit cell size and shape. Now, significant hot regions have been created exceeding 85 % of the total area. This example illustrates that deviating from the principle of equal closest distances between the wells can jeopardize the benefit of an arrangement, and lead to greater than 65% area of hot regions in the subsurface formation.

Figure 10 is a table listing examples of properties of heating well and production well arrangements with 3 heating wells per production well. The unit cell size assumes a distance of 3 m between the closest heating wells. The number of heating wells close to a production well excludes heating wells close to other production wells. Of course, the unit cell dimensions are approximate as variations will be unavoidable.

Figures 11-21 illustrates schematically exemplary arrangements of heating wells and production wells shown in cross section in a plane perpendicular to a main axis of the heating wells and production wells. There are different ratios between production wells and heating wells ranging from 1:2 to 1:6, and some characteristics of these patterns are summarized in Table 2. When the ratio of heating wells to production wells is reduced below 3 it is easy to design well patterns that do not give rise to hot regions in the reservoir, as illustrated in Figures 11 to 13. Generally, as the relative ratio of heating wells to production wells is increased, the hot region area increases as well.

Surprisngly, there are large variations in hot region proportions for a given production well to heating well ratio. The effect is best illustrated by the 1:6 production well/heating well ratio arrangements shown in Figures 20 and 21, where a hexagonal arrangement of heating wells with production wells in the center of each third hexagon gives a hot region proportion of ca. 67%, compared to rows of octagons with production well in the center where the rows can be linked together giving heating well bands of alternating pentagons, giving only 36% of the total area as hot regions. However, even for the comparably high hot region proportion of the design in Figure 19, the shortest pathway for hydrocarbons produced in the vicinity of the heating wells do not pass through a hot region, meaning reduced propensity for overheating and secondary adverse reactions compared to known configurations.

Of the four 1:4 configurations in Figures 15-18, it can be seen that the simple hexagonal arrangement in Figure 17 is less efficient compared to more complicated designs. It is interesting to compare the similar designs in Figures 19 and 21, having the same size of the unit cell and nearly the same total well configuration with the only difference being that in the 1:6 arrangement an additional well is placed in the center of the octagon. By appropriate distribution of heating wells and production wells in these patterns, it is seen that more heating wels give some increase in hot regions.

Figure 22 is a table showing features of additional heating well and production well arrangements as illustrated in Figures 11 to 21 Figure 23 is a flow diagram showing exemplary steps for arranging heating wells and production wells. The following numbering corresponds to that of Figure 23: Si. A plurality of production wells is provided in a subsurface formation.

S2. A plurality of heating wells is also provided in the subsurface formation. Note that the heating wells may heat using any suitable heating means, and may include combinations of different types of heating means. Examples of different types of heating well include electrical heaters, gas heaters, hot fluid heaters and so on.

S3. The production wells and heating wells are disposed relative to one another in a plane perpendicular to a main axis of the wells so as to form a pattern in the iO subsurface formation. Hot regions in the pattern form less than 65% of the area of the portion of the subsurface formation. Examples of patterns include close packing of wells, cubic packing of wells, interlinked pentagons of wells, pentagons of wells linked by octahedrons and truncated hexagons of wells, but it will be appreciated that other suitable patterns may be used. Note that the pattern may change along the length of the wells depending on the geology and shape of the subsurface area to be heated.

S4. The subsurface formation is heated using the heating wells.

S5. Hydrocarbons are produced using the production wells.

Figure 24 is a flow diagram showing exemplary steps for determining an arrangement of heating wells and production wells. The following numbering corresponds to that of Figure 23: S6. A preferable ratio of heating wells to production wells is determined.

S7. A unit cell describing a two-dimensional array of heating wells and production wells in the preferred ratio is determined. Next-neighbour distances between the wells are substantially equal, and the use of regular geometrical shapes of wells is preferred.

Each heating well is located in a defined position relative to a production well, the unit cell being defined in a plane perpendicular to a main axis of the wells. The determination of the unit cell may take into account factors such as a reservoir model, which may indicate that one type of unit cell is preferable to another.

SB. For the unit cell, a determination is made of the hot region area. Note that in an optional embodiment, it a group of heating and production wells in the unit cell comprises a substantially regular polygon having at least four corners, with a production well occupying one of those corners, the group of heating and production wells forming the regular polygon is not considered to contain a hot region area. This is because there is a free passage of produced hydrocarbon towards the production well.

S9. If the hot region area is less than a desired amount, then the procedure proceeds at step Sli (or step S12 if step Sil is not required). If the hot region area is higher than a desired amount then the procedure proceeds at step Sb.

510. The positions of at least one of the heating wells and or production wells relative to the other heating wells and production wells is altered in the unit cell. The procedure reverts to step SB. It may be that after a certain number of iterations, the hot region area still is higher than the desired value. In this case, the procedure reverts to step S7 and a new unit cell is defined.

Si 1. As an optional step, a determination may be made to ascertan whether a pathway between a heating well and an adjacent production well traverses a hot region. If so, then the procedure reverts to step Sb.

S12. A suitable unit cell pattern has been found that has a hot region area below a desired amount.

Note that in altering the positions of heatng wells and production wells in step Si 0 may cause a new unit cell to be defined Figure 25 illustrates schematically in a block diagram an exemplary computer device 1.

The computer device 1 is provided with a user input device 2 for receiving a value of a desired ratio of heating wells to production wells. This may be, for example, a keyboard or mouse that allows a user to select a desired ratio. A processor 3 is provided for determining a unit cell of heating wells and production wells in the desired ratio such that each heating well is located in a defined position relative to a production well, the unit cell being defined in a plane perpendicular to a main axis of the wells.

The processor is further arranged to (i) determine, for the unit cell, the hot region area.

The processor is also arranged to (H) determine it the hot region area is higher than a desired hot region area. If so, the processor alters the position of at least one of the heating wells and production wells relative to the other heating wells and production wells in the unit cell, and repeats determinations (i) to (H). The processor 3 may also determine a pathway between each heating well and an adjacent production well, and if the pathway traverses a hot region, repeat steps (i) and (H).

A non-transitory computer readable medium in the form of a memory 4 is provided that may be used for storing a program 5 and data 6. The program 5, when executed by the processor 3, causes the processor to perform the operations described above.

An output 7 may also be provided showng the final configuration of heatng wells and production wells in the unit cell.

Note that the program may be provided on an external non-transitory computer readable medium in the form of a second memory 8. The program may be transferred from the second memory B to the memory 4 or executed directly by the processor 3.

Note that in an optional embodiment, three heating wells occupy the corners of a regular polyhedron that also contains a production well. This region may not be considered to be a hot region, because in this case produced carbon can move towards the production well without coming close to any of the three heating wells.

It will be appreciated by the person of skill in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention as defined in the appended claims.

Claims

Claims 1. A method of determining an arrangement of heating wells and production wells in a subsurface formation, the method comprising: (i) determining a desired ratio of heating wells to production wells; (H) determining a unit cell of heating wells and production wells in the desired ratio such that each heating well is located in a defined position relative to a production well, the unit cell being defined in a plane perpendicular to a main axis of the wells; (Hi) determining for the unit cell a hot region area, the hot region area comprising a region in which three heating wells are closer to the hot region than a production well; (iv) in the event that the hot region area is higher than a desired hot region area, altering the position of at least one of the heating wells and production wells relative to the other heating wells and production wells in the unit cell, and repeating steps (H) to (iv).
2. The method according to claim 1, further comprising determining a pathway between each heating well and an adjacent production well, and if the pathway traverses a hot region, repeating steps (H) to (iv).
3. The method according to claim 1 or 2, wherein altering the position of at least one of the heating wells and production wells relative to the other heating wells and production wells in the unit cell causes a new unit cell to be defined.
4. The method according to any one of claims 1 to 3, further comprising, in the event that a group of heating and production wells forms a substantially regular polygon having at least four corners, with a production well occupying one of the corners, determining that the group of heating and production wells does not contain a hot region area.
5. A computer device arranged to determine an arrangement of heating wells and production wells in a subsurface formation, the computer device comprising: a user input device for receiving a value of a desired ratio of heating wells to production wells; a processor for determining a unit cell of heating wells and production wells in the desired ratio such that each heating well is located in a defined position relative to a production well, the unit cell being defined in a plane perpendicular to a main axis of the wells; the processor being further arranged to (i) determine for the unit cell a hot region area, the hot region area comprising a region in which three heating wells are closer to the hot region than a production well; the processor being further arranged to (H) determine if the hot region area is higher than a desired hot region area, and, if so, altering the position of at least one of the heating wells and production wells relative to the other heating wells and production wells in the unit cell, and repeating determinations (i) to (H).
6. The computer device according to claim 5, wherein the processor is arranged to determine a pathway between each heating well and an adjacent production well, and if the pathway traverses a hot region, repeat steps (i) and (H).
7. The computer device according to claim 5 or 6, wherein processor is arranged to alter the position of at least one of the heating wells and production wells relative to the other heating wells and production wells such that a new unit cell is defined.
8. The computer device according to any one of claims 5, 6 or 7, the processor being further arranged to determine that if a group of heating and production wells comprises a substantially regular polygon having at least four corners, with a production well occupying one of the corners, determining that the group of heating and production wells does not contain a hot region area.
9. The computer device according to any one of claims 5 to 8, wherein the processor is arranged to at least in part, use a reservoir model to determine the unit cell.
10. A computer program comprising computer readable code which, when run on a computer device, causes the computer device to perform the steps of: (i) determining a desired ratio of heating wells to production wells; (H) determining a unit cell of heating wells and production wells in the desired ratio such that each heating well is located in a defined position relative to a production well, the unit cell being defined in a plane perpendicular to a main axis of the wells; (Hi) determining for the unit cell a hot region area, the hot region area comprising a region in which three heating wells are closer to the hot region than a production well; (iv) in the event that the hot region area is higher than a desired hot region area, altering the position of at least one of the heating wells and production wells relative to the other heating wells and production wells in the unit cell, and repeating steps (H) to (iv).
11. A computer program product comprising a non-transitory computer readable medium and the computer program according to claim 10, wherein the computer program is stored on the computer readable medium.
12. A system for producing hydrocarbons from a subsurface formation, the system comprising: a plurality of production wells, at least part of each production well located in a portion of the subsurface formation; a plurality of heating wells, at least part of each heating well located in a portion of the subsurface formation; wherein the production wells and heating wells are disposed to form a pattern in the subsurface formation such that hot regions form less than 65% of the area of the portion of the subsurface formation, each hot region comprising a region in which three heating wells are closer to the hot region than a production well.
13. The system according to claim 12, wherein a group of heating and production wells forming a substantially regular polygon having at least four corners, with a production well occupying one of the corners does not contain a hot region area.
14. The system according to claim 12 or 13, wherein hot regions form ess than any of 50% of the area of the portion of the subsurface formation, 35% of the area of the portion of the subsurface formation, and 25% of the area of the portion of the subsurface formation.
15. The system according to any one of claims 12 to 14, wherein the pattern of heating wells and production wells in a cross section perpendicular to a main axis of the wells forms a repeatable unit cell within the subsurface formation.
16. The system according to claim 15, wherein the unit cell substantially takes the shape of any of a triangle, a rectangle, a pentagon, a hexagon, a rhombus and a parallelogram.
17. The system according to any one of claims 15 or 16, wherein a largest dimension of the unit cell is any of: less than fifty times an average distance between adjacent heating wells; less than twenty times an average distance between adjacent heating wells; and less than ten times an average distance between adjacent heating wells.
18. The system according to any one of claims 15, 16 or 17, wherein a sum of distances from a production well in the unit cell to all adjacent heating wells closer than a closest adjacent production well is less than a peripheral distance of the unit cell.
19. The system according to any one of claims 12 to 18, wherein the pattern of heating wells and productions wells is substantially any of: close packing of wells; cubic packing of wells; interlinked pentagons of wells; pentagons of wells linked by octahedrons; truncated hexagons of wells.
20. The system according to claim 19, wherein a production well is located at an interstitial site within a pattern of heating wells.
21. The system according to claim 20, further comprising a second pattern at a different region of the subsurface formation.
22. The system according to any one of claims 12 to 21, wherein a distance between neighbouring wells is substantially uniform throughout the pattern.
23. The system according to any one of claims 12 to 22, wherein a ratio of heating wells to production wells is between 2 and 7.
24. The system according to any one of claims 12 to 23, wherein the heating wells are arranged to heat the surrounding formation to a temperature sufficient to crack and/or pyrolize kerogen.
25. The system according to claim 24, wherein the temperature is in the range of 100cCto4500C.
26. The system according to any one of claims 12 to 24, wherein the heaters are arranged to provide heat using any of hot fluid injection, closed loop hot fluid, electrical heating, gas heating, and induction heating.
27. The system according to claim2b, wherein the heaters are arranged to provide heat using any of hot fluid injection and the heating fluid comprises any of a molten salt and an ionic liquid.28. A method of producing hydrocarbons from a subsurface formation, the method comprising: providing a plurality of production wells, at least part of each production well located in a portion of the subsurface formation; providing a plurality of heating wells, at least part of each heating well located in a portion of the subsurface formation; wherein the production wells and heating wells are disposed to form a pattern in the subsurface formation such that hot regions form less than 65% of the area of the portion of the subsurface formation, each hot region comprising a point at which three heating wells are closer to the hot region than a production well; and heating the subsurface formation using the heating wells and producing hydrocarbons using the production wells.
28. The method according to claim 27, further comprising disposing the production wells and heating wells such that hot regons form less than any of: 50% of the area of the portion of the subsurface formation; 35% of the area of the portion of the subsurface formation; and 25% of the area of the portion of the subsurface formation.
29. The method according to any one of claims 27 and 28, further comprising disposing the production wells and heating wells such that the pattern of heating wells and productions wells is substantially any of: close packing of wells; cubic packing of wells; interlinked pentagons of wells; pentagons of wells linked by octahedrons of wells; truncated hexagons of wells.
30. The method according to claim 29, further comprising disposing a production well at an interstitial site within a pattern of heating wells.
31. The method according to any one of claims 29 or 30, further comprising disposing a second pattern at a different region of the subsurface formation.
32. The method according to any one of claims 27 to 31, further comprising using the heating wells to heat the surrounding formation to a temperature sufficient to crack and/or pyrolize kerogen.
33. The method according to claim 32, wherein the temperature is in the range of 100CC to 450°C.
34. The method according to any one of claims 27 to 33, wherein at a group of heating and production wells in the unit cell forming a substantially regular polygon having at least four corners, with a production well occupying one of the corners of the polygon does not contain a hot region area.