Additives for oil recovery from reservoirs

The invention relates to the use of quaternary polyamines having formula (I) for the recovery of oil from reservoirs formula (I), wherein n>=1 R1 represents H, CH3, CH2CH3, CnH2n+1 R2 represents H2CHOHCH2, CH2, CH2CH═CH2 R3 represents CH3, CH2CH3, CnH2n+1 R4 represents CH2, CH2CH═CH2 when R2 and R4 are allyl group, they can jointly form aliphatic cyclic structures, the ratio between the carbon atoms and nitrogen atoms (C/N) ranges from 2 to 30.

The present invention relates to the use of additives capable of increasing the recovery of oil from reservoirs preferably carbonate and with a low permeability.

More specifically, it relates to the use of additives capable of modifying the wettability of the rock from oil-wet to water-wet without causing a lowering in the interfacial tension (oil-wet refers to a surface substantially wet by oil, vice versa water-wet refers to a surface predominantly wet by water).

About half of the known oil reservoirs in the world are of the carbonate type. The particular nature of these reservoirs together with the fact that they are generally fractured, have a low permeability and are oil wet, makes it difficult to recover the oil and the yields are generally much lower than 30%.

This is because the recovery process only involves the crude oil present in the fractures, whereas the oil contained inside the rock matrix is withheld as a result of the negative capillary pressure due to the oil-wet characteristics of the porous matrix.

The predominantly oil-wet nature of carbonate reservoirs is the result of physico-chemical interactions which have taken place over the years between the hydrocarbons and the rock surface and, in particular, the interaction between the carboxylic components present in the crude oil, which can be quantified with the acid number of the oil, and the rock surface. From this consideration, it follows that the re-establishment of the surface to the original water-wet conditions can be obtained by simply favouring the release of these components.

From the mid-nineties' onwards, numerous laboratories have directed their studies towards a search for additives which, when added to the injection water, are capable of increasing oil recovery from carbonate reservoirs.

All the methods so far developed are based on the use of surface-active agents or polymeric surface-active agents some of which have proved to be capable of inverting the wettability of the rock so that the entrapped oil can be spontaneously released.

One of the characteristics of surface-active agents however is that they decrease the oil-water interfacial tension which, by an imbibition process, leads to a great reduction in the capillary pressure, the driving force of the phenomenon.

This results in a slow imbibition kinetics and this is why, as of today, the industrial use of these additives has slowed down considerably.

Even if the problem has been recognized by experts in the field, a solution has not yet been found.

It has now been found that a particular group of additives, unlike what occurs with surface-active agents, is capable of modifying the wettability of the rock without lowering the water-oil interfacial tension.

In this way, a capillary pressure inversion is favoured, maintaining its absolute value high, and a spontaneous release of the oil is obtained with much higher recoveries than those so far obtained.

Two magnitudes such as the interfacial tension and the wettability are therefore treated as independent variables whereas with the approach so far adopted which envisaged the use of surface-active agents, the two magnitudes were necessarily combined.

This aspect is of particular importance considering that in recent years numerous research laboratories have directed their efforts towards the search for additives capable of favouring oil recovery from carbonate reservoirs without succeeding in identifying additives which do not lower the interfacial surface.

Bearing this in mind, an object of the present invention relates to the use of quaternary polyamines, having formula (I) for the recovery of oil from reservoirs

The ratio between the carbon atoms and nitrogen atoms (C/N) ranges from 2 to 20, and even more preferably from 2 to 12.

n is preferably >=2.

EXAMPLES

The results of the tests are indicated in tables 2 and 3 and were carried out according to the procedures described in test 1 and test 2 specified below.

In particular, the objective of test 1 is to effect a rapid qualitative screening of the additives examined, to subsequently better evaluate the performances of the most interesting products by means of quantitative analysis effected with test 2. As the latter is longer and more laborious, it was only carried out on the best additives.

Test 1: Qualitative evaluation of the additives on the efficacy of the removal of crude-oil from pretreated powders. The test is developed in two phases:

a. Aging of the Carbonate Powder

The test consists in pretreating the powder for 10 days at a constant temperature (80° C.) in the oil C. The aging is effected in steel cylinders kept under rotation.

At the end of the aging, the samples were filtered on a filter paper and left to dry. The aged powder (FIG. 1) was weighed (4 g) in test-tubes and the aqueous solution (12 g) of the additives to be studied was added to this, at a concentration of 0.1%, (the pH of all the solutions tested was controlled).

The samples were stirred with a vortex for 2 minutes. The oil release was visibly observed.

Test 2: Imbibition Test

Disks of “Leccese” rock were pretreated in the oil object of the study (C) for 1 week at 80 degrees. The rock was immersed in oil under vacuum to ensure that it had been completely imbibed by the crude-oil.

The rock samples were hung by means of a basket to computerized scales.

The analysis was carried out by immerging the oil-imbibed “Leccese” rock samples in the aqueous solution of the additives to be tested, and following their weight variations in relation to time.

Interfacial Tension Measurements

Interfacial tension measurements were effected on the products which had produced good results with the tests, in order to evaluate their surface-active properties.

The measurements were carried out with three different instruments depending on the interfacial tension value to be measured. Low interfacial tension values were measured with a Spinning Drop (Data Physics), vice versa a tensiometer (Kruss) and Pendant Drop (Data Physics) were used for values higher than 2 mN/m.

Following the procedure described in test 1, the oil removal capacity of the compounds indicated in Table 2 was evaluated.

Table 2 indicates the results relating to the main surface-active agents tested compared with water, a solution of sodium carbonate, and ethylenediammonium chloride (and the relative base) representative of a simple molecule nitrogenated with non-polymeric quaternary nitrogen.

It can be observed how only the two cationic surface-active agents (DTAB and in particular CTAB) are capable of removing the oil.

The results show that under strong oil absorption conditions on the rock (such as those obtained in the test) the mechanisms involved in the oil removal are not the lowering of the interfacial tension or the solubilization of the oil inside the micelles, but the positive charge of the cationic surface-active agents. In this respect, it should be remembered that it has recently been demonstrated that cationic surface-active agents allow a better oil recovery with respect to the other groups of surface-active agents. It has been assumed that the cationic surface-active agent is able to complex the acid components adsorbed on the rock and remove them from this, re-establishing a water-wet surface, through an ionic-couple mechanism.

According to the procedure described in test 1, the oil removal capacity of polymeric compounds differing in charge and molecular weight, was evaluated.

A series of products of 3F Chimica (Floc) was analyzed and compared with two polymers of Floger (DP-PT, DP-FO). The cationic polymers are generally used as deflocculants for the water clarification. The characteristics of the products studied are indicated in Table 3, these are polyacrylamides, cationic polyamines (and a polydiallyldimethylammonium chloride polymer, called PolyDADMAC).

Table 3 indicates the compounds tested and the result of the test.

From Table 3 it can be seen that only a few cationic polyamines are capable of removing the crude oil, and in particular a polymer is indicated, Floc 572 (polydimethyldiallylammonium chloride,FIG. 3) which gives a highly positive result to the test.

The photo ofFIG. 3shows the oil removal capacity on the part of Floc 572 compared with surface-active agents. The photo also shows that the cationic polymer DP/PT does not work.

The photo ofFIG. 4refers to a comparison between the solutions of CTAB and the polymer Floc 572 at 0.1%. The photo shows the different performances of the two additives marked by a different interfacial tension of their aqueous solutions: the surface-active agent in fact dissolves the oil inside the micelles giving rise to the formation of a dark solution, vice versa the polymer clearly separates the oil.

Table 4 indicates the interfacial tensions of Floc 572 compared with CTAB. It can be seen that the polymer does not have surface-active properties, in fact, it slightly lowers the surface tension of the water and interfacial tension with heptane and the crude-oil A.

Test 2 was carried out on both additives in order to quantitatively evaluate the performances of Floc 572 compared with the cationic surface-active agent CTAB.

The imbibing capacity of CTAB and Floc 572 is evaluated operating according to the procedure described in test 2.

The results of the tests are clearly indicated inFIG. 5. The ordinate indicates the weight variation with respect to the initial dry weight. It can be observed that when the rock is immersed in water, a slight decrease in its weight is measured, indicating the fact that the water acts by removing the excess oil. Vice versa, both the surface-active agent and the polymeric solution cause an increase in the relative weight, if the former produces a marginal effect, however, the latter causes a variation of 2.8%.

According to the procedure described in test 2, the imbibing capacity of a solution of Floc 572 in KCl 2% is evaluated compared with the same saline solution containing 0.1% of CTAB.

The results obtained are analogous to those obtained in Example 29. The test again showed the greater efficacy of the polymer in the oil substitution and confirms that a greater capillary pressure induced by the high interfacial tension allows an easier imbibition of the rock, leading to a greater oil recovery.