EP3857024A1 - Method for producing mineral oil from a fractured underground mineral oil reservoir - Google Patents

Abstract

The present invention relates to a method for producing mineral oil from a fractured underground mineral oil reservoir, the reservoir comprising a rock matrix penetrated and surrounded by fractures, the reservoir further comprising an aquifer below an oil containing zone with an oil-water-contact zone (OWC) formed between the aquifer and the oil containing zone, where at least one injection well has been sunk into the aquifer and at least one production well has been sunk into the oil containing zone. The method comprises at least the steps of (a) injecting a surfactant composition into the aquifer below the OWC through the injection well, the surfactant composition having a lower density than the water in the aquifer, causing the surfactant composition being driven into the oil containing zone, and (b) recovering mineral oil from the production well.

Description

Method for producing mineral oil from a fractured underground mineral oil reservoir Description

The present invention relates to a method for producing mineral oil from a fractured under ground mineral oil reservoir, the reservoir com prising a rock matrix penetrated and su rrou nded by fractures, the reservoir further com prising an aquifer below an oil containing zone with an oil-water-contact zone (OWC) formed between the aquifer and the oil containing zone, where at least one injection well has been su nk into the aquifer and at least one production well has been sunk into the oil containing zone.

Naturally fractu red reservoirs contain a significant part of the world’s oil reserves, represented in two interrelated systems: rock matrix and fractu res. I n general, rock matrix blocks are penetrated and su rrounded by fractures that occupy on ly a small percentage of the total reservoir volu me. The permeability of the rock matrix is significantly lower than the permeability of the fractu res. T ransport of oil occurs via the network of high ly permeable fractures receiving the oil from the matrix blocks acting as a sou rce of hydrocarbons. Apart from primary depletion, oil residing in the matrix blocks can be transferred into the fractures by water or gas displacement. This interaction between fractures and rock matrix is governed by a pressu re gradient imposed du ring the water flow and spontaneous penetration (imbibition) of water into the matrix blocks, owing to capillary and gravity forces. The presence of highly permeable fractu res limits the buildup of sufficient pressu re drop or viscous forces across the matrix blocks. Therefore, the main fluid exchange between the fracture network and matrix blocks occur due to spontaneous imbibition. The natu ral occurrence and rate of spontaneous imbibition depends on several influencing factors like fracture density and geometry, matrix block size and wettability of the matrix blocks.

Producing oil from such fractured reservoirs is a challenging task due to the highly conductive fracture network that makes the fluid flow significantly different from that observed in unfractu red reservoirs. Negative capillary pressure and high permeability contrast retains the oil in matrix blocks and makes the oil recovery by primary and secondary methods inefficient.

Chemically Enhanced Oil Recovery (EOR) as a so-called“tertiary oil production method” aims at affecting the reservoir fluids and rock properties in order to increase oil production. The application of surfactants as chemical EOR agents in fractured reservoirs has received an increased attention within the last decades. The recovery of oil by surfactants in the fractured reservoirs is based on the invasion of the su rfactant from the fractu re into the matrix and either a reduction of the interfacial tension which enables gravity to displace the oil from the bottom to the top face of the matrix, or alteration of the rock wettability towards water wet, when the capillary pressu re is driving the oil recovery from the matrix.

The docu ment US 3,548,941 for example discloses methods for using surfactants for recovering oil from a fractu red matrix reservoir with a special focus on cyclically changing the concentration of su rfactant that is injected into the reservoir. As can be seen from that document, the strategy how to inject a su rfactant solution is crucial for the efficiency of oil recovery by surfactant-based EOR approaches.

The docu ment“Mohammad Amin Gholamzadeh et at.: Optimization of Injection Rate and I njection Time for Surfactant and Water Flooding in Oil Fields” (International Journal of Advancements in Research & Tech nology, Volume 1, Issue 3, August 2012) presents simulation results of the effects of water flooding and su rfactant flooding on oil production in conventional reservoirs and in fractured reservoirs. According to the authors, well placement plays an important role on the achievable oil recovery rate.

Fractu red underground mineral oil reservoirs with an u nderlying aquifer represent a special case of fractu red reservoirs. Surfactant-based EOR methods are difficult to realize in this type of reservoirs due to permeability differences between fractu res and rock matrix. Even though there are methods for the application of surfactants as EOR agents in aquifer-d riven reservoirs, efficient strategies for fractu red reservoirs with an underlying active aquifer are lacking.

US 4,372,381 discloses a method for recovering oil from a tilted oil-bearing reservoir having a water zone in fluid com mu nication with and directly below an oil-bearing zone wherein a large amount of solvent is injected along the water-oil interface such that a part of the solvent fingers into the oil, lowering the viscosity of the oil and making the oil more mobile for production. The solvent is miscible with oil and is injected into the lower portion of the reservoir in the vicinity of the oil-water interface in order to en hance the fingering of the solvent into the oil. The aim of this method is to create an interface between the water in the aquifer and the mixed oil-solvent phase and to drive the oil-solvent phase u pwards towards a production well. Apart from the fact that the introduction of a solvent that is miscible with oil does not solve the problem of oil recovery from fractured reservoirs, according to the disclosure, this approach requires a large amou nt of solvent, making this approach inefficient and expensive.

The method disclosed in US 4,838,350 is concerned with gravity stable su rfactant flooding, particularly in vertical reservoirs which have been depleted by gas drive or gas expansion recovery methods. A su rfactant slug is injected into the bottom of the reservoir above any water-flooded zone or underlying aquifer in an amount sufficient to form a thin layer or blan ket on top of any underlying water zone. A driving force is applied to drive the su rfactant slug upwards through the reservoir to create a hydrocarbon ban k above the su rfactant slug. The method is especially well suited for vertical reef reservoirs. For fractured reservoirs with significant differences in permeability between the oil-bearing rock matrix and the surrou nding fractures, this method is disadvantageous, as the injection of the su rfactant into the oil-bearing zone would lead to a very heterogeneous distribution of the surfactant in the fractures and thus to an inefficient and incom plete oil recovery.

Despite of a variety of surfactant-based EOR methods, there is a need for efficient methods to produce oil from fractu red underground mineral oil reservoirs with an underlying aquifer in tertiary oil recovery. It was an object of the invention to provide a method for producing mineral oil from aquifer- driven fractured reservoirs which increases the yield of mineral oil produced.

This object is achieved according to the invention by a method for producing mineral oil from a fractu red underground mineral oil reservoir according to claim 1. Advantageous embodiments and further developments of the invention are indicated in the dependent claims 2 to 6.

The present invention provides a method for producing mineral oil from a fractured underground mineral oil reservoir, the reservoir com prising a rock matrix penetrated and su rrou nded by fractures, the reservoir fu rther com prising an aquifer below an oil containing zone with an oil-water-contact zone (OWC) formed between the aquifer and the oil containing zone, where at least one injection well has been su nk into the aquifer and at least one production well has been sunk into the oil containing zone, the method comprising at least the following steps:

a) injecting a su rfactant composition into the aquifer below the OWC through the injection well, the surfactant composition having a lower density than the water in the aquifer, causing the surfactant composition being driven into the oil containing zone, and b) recovering mineral oil from the production well.

Reservoirs according to the invention are also referred to as“bottom aquifer-d riven fractured reservoirs”. An aquifer is understood to be an undergrou nd layer of water-bearing permeable rock, rock fractures or unconsolidated material like gravel, sand or silt. The aquifer is typically saturated with water.

The injected surfactant forms a layer between the u nderlying aquifer and the oil phase. Com pared to processes known in the prior art that suggest to inject a surfactant into the oil phase or at the OWC, injecting a surfactant com position into the aquifer below the OWC has the advantage of a more uniform distribution of the su rfactant layer being formed.

Methods to determine the OWC are known in the art, e.g. based on measurements of formation evaluation like well logging. The OWC can also be determined by numerical simulations based on the knowledge about the reservoir and/or measu rements of reservoir- specific parameters.

According to the invention the surfactant composition has a lower density than the water in the aquifer. The terms“density” and“gravity” are used interchangeably in this context. The density difference (or gravity difference) between the injected su rfactant composition and the water in the aquifer is the driving force for driving the surfactant com position into the oil containing zone. Without a density difference between the injected su rfactant composition and the water in the aquifer the su rfactant would widely spread or dissolve in the aquifer such that large amounts of su rfactant would be necessary due to losses of surfactant into the aquifer. I njecting the surfactant com position with a lower density than the water into the aquifer below the OWC assures that only a small amount of su rfactant is needed to d rive an essentially uniform surfactant slug into the fractures of the reservoir.

Depending on the kind of reservoir, an appropriate surfactant or mixtu re of su rfactants can be chosen. For reservoirs where the rock is either oil wet or mixed to oil wet, water spontaneous imbibition into the rock matrix blocks is rather limited due to negative capillary threshold pressure and on ly the oil residing in the fractures will be produced du ring the waterflooding.

I n one embodiment, surfactants are injected that reduce the interfacial tension, e.g. anionic su rfactants. The surfactant penetrates the fractures and reduces the interfacial tension between the oil and water phases causing the negative capillary pressure to approach zero. As soon as the capillary pressu re d rops below the negative th reshold pressure, the gravity forces initiate the gravity drainage of water into the matrix block. Water starts imbibing from the bottom in a piston-like manner and displacing the oil by co-current flow from the top of matrix block.

I n a fu rther embodiment, surfactants are injected that alter the wettability of the rock matrix blocks, e.g. cationic su rfactants. The su rfactant adsorbed on the rock surface alters the rock wettability towards water-wet changing the sign of capillary pressure from negative to positive. Alteration of wettability in oil wet rock causes the capillary imbibition of water into the matrix block from all su rfaces of the matrix block. I mbibition is driven by positive capillary pressu re and water imbibes counter-currently displacing the oil in opposite direction. Injecting wettability altering surfactants also equilibrates the water level in fractures and matrix blocks by d raining the oil out of the matrix blocks.

I n a fu rther embodiment of the invention, a combination or mixture of surfactants is injected that in effect reduces the interfacial tension and alters the wettability, e.g. a combination or mixtu re of anionic and cationic surfactants.

The mineral oil displaced out of the rock matrix block into the fractures due to gravity segregation forms an oil bank flowing on top of the water phase flowing u pwards from the aquifer. The surfactant slug supports the formation of an oil ban k being pushed in vertical direction by encroaching water from the aquifer.

I njecting a surfactant composition with a lower density than the water into the aquifer below the OWC assures that the surfactant flows into the fractu red matrix reservoir and induces gravity forces that counteract the rapid penetration of the high ly permeable fractures by water from the aquifer. This method prevents the early water breakthrough into the production well.

I n a preferred embodiment the surfactant com position has a lower salinity than the water in the aquifer. The lower salinity leads to a lower density of the surfactant com position. The lower density gives rise to driving forces that drive the surfactant composition into the oil bearing zone and prevent the surfactant to mix or dissolve in the aquifer.

I n another preferred embodiment the surfactant com position has a higher tem perature than the water in the aquifer. The elevated temperature reduces the density of the su rfactant composition compared to that of the water in the aquifer.

I n a fu rther preferred embodiment the surfactant com position has a lower salinity and a higher temperature than the water in the aquifer. This has a synergistic effect on the driving forces that drive the surfactant composition into the oil-bearing zone.

I n another preferred embodiment the surfactant composition is horizontally injected into the aquifer below the OWC. The horizontal injection supports the uniform distribution of the su rfactant layer being formed. I n combination with the lower density compared to the water in the aquifer the surfactant composition mainly spreads in horizontal directions and merely mixes or dissolves in the water of the aquifer.

I n a preferred embodiment the flow rate of the injected surfactant is determined as a function of the degree of dispersion of the su rfactant in the aquifer. The injection flow rate of the surfactant com position can be determined by numerical simulation of the

undergrou nd reservoi r. The degree of dispersion can be determined by numerical simulation and/or according to methods based on measu rements known in the art, e.g. by tracer tests in the reservoir.

I n a further preferred embodiment the flow rate of the injected surfactant is determined such that an uniform surfactant slug is created in the aquifer below the OWC. A uniform su rfactant slug is advantageous as it provides a high area of contact to the liquid phase in the fractures and as it prevents the water from the aquifer from breaking th rough single fractures.

The method according to the invention provides several advantages compared to processes known in the prior art for fractured reservoirs. The amou nt of su rfactant necessary to build up and drive a surfactant slug through the oil-bearing zone is minimized, thereby reducing operating costs and minimizing environmental im pact of oil production. Due to a more uniform surfactant slug, water break-th roughs are minimized or even completely prevented, increasing the amount of oil produced from the reservoir and thus the efficiency of the oil production method.

Claims

1. A method for producing mineral oil from a fractured underground mineral oil reservoir, the reservoir comprising a rock matrix penetrated and surrounded by fractures, the reservoir further comprising an aquifer below an oil containing zone with an oil-water- contact zone (OWC) formed between the aquifer and the oil containing zone, where at least one injection well has been sunk into the aquifer and at least one production well has been sunk into the oil containing zone, the method comprising at least the following steps:

a) injecting a surfactant composition into the aquifer below the OWC through the injection well, the surfactant composition having a lower density than the water in the aquifer, causing the surfactant composition being driven into the oil containing zone, and

b) recovering mineral oil from the production well.

2. The method according to claim 1, wherein the surfactant composition reduces the interfacial tension between the oil and water phases.

3. The method according to claim 1 or 2, wherein the surfactant composition alters the wettability of the rock matrix blocks.

4. The method according to any one of claims 1 to 3, wherein the surfactant composition has a lower salinity than the water in the aquifer.

5. The method according to any one of claims 1 to 4, wherein the surfactant composition has a higher temperature than the water in the aquifer.

6. The method according to any one of claims 1 to 5, wherein the surfactant composition is horizontally injected into the aquifer below the OWC.

7. The method according to any one of claims 1 to 6, wherein the flow rate of the

injected surfactant is determined as a function of the degree of dispersion of the surfactant in the aquifer.

The method according to claim 7, wherein the flow rate is determined such that an uniform surfactant slug is created in the aquifer below the OWC.