Method for stabilizing clay minerals during oil exploitation by steam injection

The present invention relates to a new process for stabilizing the clay minerals in reservoir rocks containing oil, in particular in rock comprising sandstones, sand, or marliceous sandstones as well as eliminating the electrostatic dispersion and swelling of the clays when steam injection is used for mineral oil exploitation. According to the process, cations capable of stabilizing the reservoir rock, having a hydrated diameter of 0.13 to 0.15 nm, preferably potassium and/or ammonium and/or zirconium ions are introduced in the form of a 0.01 to 5.0 N aqueous solution or in the form of a vapor mixture or a mixture of steam and gas(es).

TECHNICAL FIELD 
The present invention relates to a new process for stabilizing the clay 
minerals in oil-containing reservoir rocks, in particular in rocks such as 
sandstones, sand, or marliceous sandstones, as well as for eliminating 
electrostatic dispersion and swelling of the same when steam injection is 
used for oil exploitation. 
BACKGROUND ART 
The so-called thermic methods represent a significant group of secondary 
and tertiary methods of oil expoitation, in the course of which the 
highest possible part of oil reserves which could not be exploited 
traditionally are attempted to be mobilized partly by oxidation in situ, 
partly by injecting steam or vapor mixtures. 
In the specification of the present invention, the scope is restricted to 
the injection of steam, a mixture of vapor, steam containing carbon 
dioxide and mixtures of vapors containing carbon dioxide; within this 
far-reaching field which includes a plurality of technical problems, we 
are particularly dealing with the behavior of the clay minerals of 
reservoir rocks, as they play a most determinant role in defining the 
hydrodynamic state of reservoir rocks. 
It is a well known fact that clay minerals with a layer structure of 2:1, 
e.g. the montmorillonites, are capable of considerable hydration, as a 
consequence of which the size of the cells increases in the direction of 
the C-axis, their stability decreases, and their dispersion begins. In the 
course of clay dispersion, the content of the suspended material of fluids 
(in particular water and oil) flowing in the interconnected pore system of 
the rock increases, which accumulates in certain parts of the rock (due to 
mechanical filtration or for colloid-chemical reasons) and forms local 
flow barriers, thus deteriorating the average permeability and increasing 
detrimental heterogeneity. Swelling and dispersion are especially harmful, 
if the ionic character of the rock determined by the original fluids, 
i.e., the natural state of equilibrium, is disturbed by relatively drastic 
methods, such as steam injection. 
It is quite obvious, that not only the state of the clay minerals but also 
the thermodynamic conditions of the whole environment--in particular, in 
zones surrounding the well--are changed, while these changes exert a 
considerable effect, generally disadvantageous changes on the hydrodynamic 
properties of the porous system, as a consequence of the modified 
permeability of the rock, as described hereinabove. 
If steam or steam also containing organic compounds is injected, it is 
obvious that a more or less superheated steam phase has to be injected. 
However, in this way condensation within the rock, i.e., quick and 
considerable dilution of the aqueous phase cannot be avoided either, so 
the original ionic character diminishes. As water molecules can be 
incorporated easily into the clay minerals, with a high energy, the 
natural ion content of the clays is exchanged, the clays begin to swell 
and become dispersed; this phenomenon can result--in particular with rocks 
with a high clay content--in the collapse of the structure, up to the 
collapse of the zones in the environment of wells. 
Summing up what has been said, in addition to other factors, inhibition, 
elimination of changes in clay minerals, the so-called clay-effect is of 
utmost importance. 
SUMMARY OF THE INVENTION 
The present invention relates to a method of stabilizing clay minerals of 
oil reservoir rocks, in particular in the case of rocks containing sands, 
sandstones and marliceous sandstones when exploitation of oil is performed 
by steam injection. First of all, clay minerals capable of swelling and 
having a layer structure of 2:1 are stabilized prior to and/or 
simultaneously with steam injection with cations introduced into the 
reservoir rock, said ions having a hydrated diameter of 0.13 to 0.15 nm 
and a coordination number 12, which means that the swelling capability of 
the clays is inhibited; in the course of said treatment the cations, 
having been introduced in an aqueous solution or steam phase into the 
reservoir, rock are incorporated into the inner structure of the clay 
minerals and/or are fixed therein, thus reducing the size of the unit cell 
of the clay minerals in the direction of the C-axis, and also reducing the 
inner hydrate contents, porosity, sorption capacity and heterogeneity. At 
the same time, the permeability of the rock is increased and stabilized 
during the course of injecting the steam. In such a manner, steam can be 
injected undisturbed, with less expenditure of energy, thus forming the 
prerequisite of efficient application of thermic exploitation of 
hydrocarbons, when steam is injected. 
DETAILED DESCRIPTION OF THE INVENTION 
During our experiments we found that the cell size of clay minerals, in 
particular those tending to swell and having a layer structure of 2:1, 
e.g., montmorillonite, bentonite, nontronite, vermiculite, etc. in the 
direction of the C-axis can be reduced. Additionally, the Si-Al layers can 
be brought nearer to each other with high energy. If cations are 
introduced into the aqueous environment, which become fixed in the layer 
structure by exchanging the Na.sup.+, Ca.sup.2+, Mg.sup.2+ ions of the 
clay. Thus, they considerably increase the electrostatic binding energy of 
the layers, i.e., the clays are stabilized, and simultaneously dispersion 
is inhibited. For this purpose cations are preferably used with a hydrated 
diameter of 0.13 to 0.15 nm and with a coordination number 12, the salts 
of which are soluble in water and dissociate to the required extent, e.g., 
different potassium and ammonium salts, and some zirconium salts. 
In several cases it can be misleading that the clay content capable of 
swelling in certain sandstones or marliceous sandstones, seems to be 
negligible in comparison to the total mass of the rock, e.g., not more 
than 3 to 5%. In other words, the clay is present in a dispersed state, so 
the role and effect to be expected are not taken into account. However, it 
can be demonstrated that grains of the clay minerals play a role of utmost 
importance in consolidation of the other mineral rock-forming components, 
in the natural cementing thereof. Accordingly, the effect of the clays 
does not reside merely in their own dispersion but they also can influence 
the stability of the whole rock. Their own cementing role ceases in the 
course of dispersion, while the original permeability of the rock 
drastically changes. 
In connection with methods based on the injection of steam, mixtures of 
vapors, steam and gaseous media, e.g. CO.sub.2, we wish to call attention 
to the fact& that the sorption features of the loosened dispersed clays 
are negatively changing, they increase considerably. Considering the fact 
that certain organic groups and molecules may be also incorporated into 
the clays, frequently with a higher swelling energy than water, 
displacement of solid surfaces in the oil-wet direction can be really most 
disadvantageous. 
We were able to demonstrate that, due to the effect of the inhibiting 
cations, the dispersion of clays considerably decreases. Moreover, 
depending on the concentration, it can fully stop, so that by the 
inclusion thereof, advantageous conditions can be obtained for steam 
injection. 
Treatment of the reservoir rock can be performed in such a way that, prior 
to steam injection, the cations selected for the treatment are introduced 
into the layer in the form of an aqueous solution with a concentration in 
the range between 0.01 to 5.0 N preferably 0.05 and 3.0 N. The aqueous 
solution is introduced as a slug of a size of 0.005 to 0.7, preferably 
0.05 to 0.5, calculated on the pore volume of the rock situated between 
the proding well and the injecting bore. Thereafter the system is allowed 
to rest in order to restore the thermal equilibrium. The period of rest is 
followed by injecting the steam. As a consequence of steam injection, the 
thermal state of the injected tract of the reservoir rock changes, the 
temperature rises and, as a result, inhibiting cations are incorporated at 
an accelerated rate, since--according to our experience--the process or 
ion-exchange fixation is an endothermic process. 
The temperature of the injection of the steam, vapor mixtures or 
steam/gaseous mixtures, as noted above, is about 190.degree.-300.degree. 
C., preferably 200.degree.-300.degree. C. at reservoir pressures. 
Together with the steam phase, inhibiting cations may be introduced into 
the rock to be treated in the form of any compound which is in the vapor 
phase at the temperature of steam injection; this method is well 
applicable even if the compound undergoes thermal dissociation. However, 
later, in the condensed phase in water the ions of the original compound 
should be unchanged. In these cases, for inhibiting the clay effect, e.g., 
volatile ammonium salts, such as ammonium hydroxide, ammonium carbonate, 
ammonium hydrogen carbonate, etc., or volatile potassium salts, such as 
potassium formate, potassium acetate, potassium hydrogen carbonate, etc., 
can be successfully used. 
Inhibiting cations can be introduced not only prior to steam injection but 
also during the process, as an intermediate step. Moreover, inhibition 
with an aqueous phase cas be combined with inhibition with a steam phase 
in one or more treating cycles, by guaranteeing the qualitative and 
quantitative indices in compliance with the preliminary examinations on 
the properties of the given reservoir rock. 
For the treatment with inhibitors, potassiumn, and/or ammonium, and/or 
zirconium ions are applied in a concentration of from 0.01 to 5.0 N, in a 
volume of 0.005 to 0.7 in an aqueous, steam-phase or in the phase of the 
mixture of steam and gaseous medium, related to the pore volume of the 
reservoir rock situated between the wells. Depending on the peculiar 
feature of the reservoir rock, the treatment can be performed with a 
series of smaller slugs, of increasing or decreasing concentration, by 
introducing the slugs comprising the inhibiting ions, with or without 
spacer fluid slugs. 
The introduction of inhibitor ions used in the phases of steam, 
vapor-mixture or mixture of steam and gaseous medium can be performed by 
recovering the slugs consisting of the injected steam, steam/gas or the 
mixture thereof, repeating this cycle of injection/recovery more than 
once, in order to intensify the interaction between the injected fluid(s) 
and reservoir rock. 
The inhibition of the clay-effect is preferable even in the case when prior 
to the injection the heterogeneity of the reservoir rock to be injected 
with steam is balanced. 
The method of inhibition of the clay-effect--when oil is exploited by steam 
injection--yields the following advantages: 
1. Dispersion of clay minerals resulting from swelling may be restricted or 
excluded, thus the stabilization of the reservoir rock is assured. 
2. By stabilizing the rock structure, improved permeability can be achieved 
which is advantageous in further processes, e.g., to perform steam 
injection combined with the use of carbon dioxide. 
3. The injected steam will be distributed more uniformly in the reservoir 
rock, the energy expenditure is less, the injected thermal energy can be 
better utilized. 
4. The flooded reservoir area may be enlarged, heterogeneity diminishes. 
5. The sorption capacity of the rock will decrease, enabling the treatment 
with chemicals with a low loss of the chemical in the course of further 
technological steps (e.g., vapors of organic compounds otherwise resulting 
in a considerable clay swelling can be injected). 
6. Steam can be injected into reservoir rocks, into which the process could 
not be performed otherwise without damaging the layer.

EXAMPLE 1 
Steam injection was carried out at a pressure of 60 bar, at a temperature 
of 493.degree. K. with a steam of a quality of 0.8. The model rock was 
prepared from a non-consolidated rock grist of a diameter of 4.91 
cm.sup.2, of a porosity of 0.31, of a length of 100 cm and comprising 4.7% 
by weight swelling clay, by compression. 
The steam was injected at a rate of 7.5.10.sup.-2 cm.sup.3 /s, the 
depression measured at the beginning of the injection was 0.345 bar. 
After a steam injection of 1.0 Vp the depression increased to 0.476 bar due 
to the 28% decrease of the permeability of the swelling clays. 
In order to eliminate the clay-effect for the stabilization of the clay 
minerals NH.sub.4.sup.+ ions were injected into the rock in the vapor 
phase in an amount of 1.1 g/kg of swelling clay. Then the system was 
allowed to stand for 2 hours. 
After the treatment the steam injection was continued, when first 0.314 bar 
depression was measured and the depression stabilized at a value of 0.330 
bar after the injection of 1.0 Vp of steam. Due to the treatment the 
swelling of the clays was eliminated, the clays were stabilized while the 
hydrodynamic parameters of the rock were also stabilized. 
EXAMPLE 2 
The parameters of the model rock were as follows: Non-consolidated rock 
prepared by compression 
length: 100 cm 
porosity: 0.30, 
air permeability: 143 mD. 
The test pressure was 60 bars, the temperature of the steam was 493.degree. 
K., the quality of the steam was 0.8. 
Before the steam injection a 1:1 mixture of K.sup.+ and NH.sub.4.sup.+ 
ions was introduced in the vapor phase in an amount of 1.2 g/kg of 
swelling clay. Then it was allowed to stand. 
At the beginning of the steam injection at a rate of 7.54.10.sup.-2 
cm.sup.3 /s, a depression of 0.294 bar, while after a steam injection of 
1.0 Vp 0.301 stabilized depression could be measured. 
During the steam injection the decrease of the permeability of the rock 
proved to be insignificant, i.e. the swelling ability of the clay minerals 
ceased as a result of the treatment, the clays stabilized. 
Without detailing all possible variation of clay-effect inhibition it 
should be emphasized that several modifications are possible within the 
scope of the claims.