66-06

During 60^th - 70^th in the former USSR an extensive Program was carried out on peaceful application of underground nuclear explosions. The Program was similar to "Plowsher" Program carried out at that time in USA. It should be noted, that investigations carried out in the USSR covered wider spectrum of directions as compared to USA. Rather interesting results were obtained on a number of those directions.

Not dwelling on the reasons, which bear both objective and subjective characters, because of which investigations on peaceful application of underground nuclear explosions in our country and abroad are not being conducted at present, we consider it useful for specialists to present the results of such investigations on one of the directions, i.e. on construction of enlarged injection wells for underground disposal of industrial sewage. Interesting results were obtained on this direction, all the more enlarged wells were continued to be successfully operated, besides in USA similar activities were not carried out.

All the more publishing of that material from our point of view was necessary as during late years many articles appeared in press the authors of which do not possess sufficient information and mark only drawbacks in peaceful application of underground nuclear explosions. Of cause such drawbacks were in separate cases.

Choice of this direction for peaceful application of underground nuclear explosions was not accidental and connected with the fact that during late years the problem for environment protection in particular, water resources from pollution with industrial sewage became very actual.

The complexity of solution of the problem is deteriorated because of the absence of efficient methods for purification of industrial sewage, and that makes it difficult to widely introduce wasteless technologies.

Undoubtedly most radical solution of the problem consists in preventing pollution of the environment with industrial sewage by introduction of wasteless technologies everywhere. However, for many components of sewage efficient methods for purification have not been developed yet.

In connection with above-stated in our country and abroad injection of industrial sewage is widely used as one of the methods for rendering sewage harmless. Injection is carried out in deep-lying underground horizons /1,2,3/. Undoubtedly this method is not an ideal solution of the problem, however, it gives the possibility to win the time for development of more improved purification methods and for introduction of wasteless technologies.

Main merit of the method consists in the fact, that industrial sewage is completely removed from the sphere of mans activity. However, method for disposal of sewage utilizing usual wells has a number of significant drawbacks, which limit its technological possibilities and decrease technical and economic efficiency.

1. Usual wells have low probability for development of layer zones with high natural permeability and relatively low average rate of flow characteristics.

2. Usual wells have low filtration surface and accumulation capacity for accumulation of suspended particles, and as a consequence are characterized by high sensitivity to quantity and sizes of suspended particles contained in sewage. Detailed investigations, carried out by U.M.Baikov/3/, showed that depending on collector type allowable quantity of suspended particles in sewage disposed of through usual wells must not exceed 15-25 mg/1. When the content of suspended particles in sewage exceeds the value of 15-30 mg/1, it can bring to sharp decrease of acceleration characteristics of usual wells - 7-10 times and more during one month /3,4/.

Usually the content of suspended particles in sewage constitutes not less than 50-100 mg/1. Lowering of their content to above mentioned levels demands additional expenditures.

3. Through usual wells it is practically impossible to carry out disposal of sewage chemically incompatible with stratal waters.

So there is a direct evidence of the contradiction between large natural acceleration characteristics of collectors and limited possibilities of existing technology to deliver sewage to underground horizons.

On the basis of theoretical evaluations and analysis of the experience accumulated while development of oil fields it was shown that in principle technological parameters of usual wells could be somehow improved while using such known methods of intensification as treatment with hydrochloric acid, hydraulic fracturing of a layer. However, significant elimination of the drawbacks was not obtained.

Thus to cardinally improve technological parameters of the wells used for underground disposal of sewage containing large quantities of suspended particles (more than 15-30 mg/1), and also sewage incompatible chemically with stratal waters, it is necessary to utilize more effective means to act on the layer, and in this connection the decision was made to carry out experimental and industrial activities on construction of enlarged injection wells with application of underground nuclear explosions. The term "enlarged well" means the well effective radius of which exceeds the shaft radius of the drilled well. In the case under consideration this term will mean the well in the face of which an underground nuclear explosion was conducted in the permeable layer. As a nuclear explosion significantly increases permeability of the layer over a large distance from the face (tens and even hundreds of meters), it is natural that one of the consequences of such an effect on the layer results in the increase of effective radius and accordingly the rate of flow characteristics

Calculations showed that acceleration characteristics of the enlarged well for homogenous layer can 2-3 times exceed the one for usual well not taking into account the influence of suspended particles. Taking into account that the enlarged well has a huge filtration surface (hundreds of square meters) and large volume of interdebris voids in the chimney (tens of thousand m³) in which suspended particles can sedimentate and accumulate, the enlarged well must be characterized by more stable operation and low sensitivity to the content of suspended particles in sewage.

It is evident that under other equal conditions allowable norm of mechanical admixtures content in sewage injected into the enlarged well will be as much higher as filtration surface of the enlarged well is larger as compared to filtration surface in a usual well. Calculations show, that taking into account average sizes of zones of explosion effect on the collector allowable content of suspended particles in sewage disposed of through an enlarged well can exceed for an order the corresponding norms for usual wells.

Durability of stable operation of the injection well for sewage injection i.e. without significant decrease of its acceleration characteristics will be defined by volume of voids, fractures in near the face well zone in which mechanical admixtures will accumulate (precipitate) excluding particles with small sizes which are capable to pass through pores of the layer over significant distances from the well face.

Based on the experience of oil fields development it is well know, that all types of layer-collectors are characterized by inhomogeneity of permeability. Especial high degree of inhomogeneity is inherent of the collectors formed of carbonaceous rocks.

Utilizing probability theory it was shown, that the probability for development of highly permeable zone of the layer with the enlarged well can exceed for an order and more the corresponding the probability for usual wells.

From here one can understand the event often met in oil field mining practice, when the well drilled in the vicinity (at the distance of several meters) of the well characterized by high productivity or acceleration characteristics turned out to be low productive or dry. Such events occur most often while developing carbonaceous collectors.

Besides, through enlarged wells it is possible to dispose of industrial sewage chemically incompatible with stratal waters. As the face of the enlarged well is the chimney the volume of interdebris space constitutes thousands of cubic meter. Both suspended particles brought with sewage and precipitation's formed while chemical interaction of injected sewage with stratal waters can sedimentate in it. Mixture of sewage with stratal waters, which has already reacted with each other, with volume almost equalled the volume of the preliminary cavity, is displaced by coming portions of sewage into the layer and serves as natural trimming for chemically neutral liquid.

As it is known from underground hydrodynamics, that significant influence on the acceleration characteristics (debit) of wells makes the deterioration of collector properties only in the vicinity of the well face,. in best case several meters from the face. So, it can be seen that chemical incompatibility of disposed sewage with stratal waters practically will not influence on technological parameters of enlarged injection wells.

Let us mark that is one of the most important advantages of enlarged wells as disposed sewage in most cases are chemically incompatible with stratal waters.

Practical examination of the efficiency for utilization of enlarged wells for underground disposal of sewage was carried out at two test-industrial objects similar by geological structure, technical and technological solutions.

At the first object "Kama-2" introduced into operation in 1976 underground disposal of sewage from soda-cement plant is carried out. At the second object "Kama-1" introduced into operation in 1983 sewage from oil-chemical integrated works is being disposed of.

On fig.1 a schematic geological section and layout of injection and observation wells at "Kama-2" is given.

As absorbing layer-collector (working horizon) carbonaceous sediments of Vizeisko-Bashkirsky rock formation was chosen which refer to medium-low-rock coal water-bearing complex containing stratal waters with chloride-sodium composition and total mineralization of 250-270 mg/l.

Chosen working horizon has regional spreading by area, depth of roof bedding of the working horizon at the objects "Kama-2" and "Kama-1" is within the limits of 1760-1880 m, thickness - 450-480 m. Working horizon is overlapped by two regional developed water confining layers one of which is presented by clay sediments with total thickness of about 70 m, and the second - by sediments of rock salt with total thickness of 450-480 m.

Comparison of coefficients for acceleration characteristics in H-1 well before and after the explosion i.e. in a usual and in an enlarged wells showed that acceleration characteristics of an enlarged well increased three times..

Using real data on the increase of acceleration characteristics of an enlarged well which were obtained by experimental injection of liquid into H-1 well before and after the explosion the value effective radius was assessed as equalled 44,5 m. and is in a good correspondence with the value R estimated on the basis of data on mechanical action of the explosion. From here one can see, that efficient radius of an enlarged well can exceed the radius of a usual well several orders.

Sewage from soda-cement plant differ by high mineralization, contains suspended particles (50-100 mg/l) and besides, is chemically incompatible with stratal waters. Special investigations showed that when sewage is mixed with stratal water solid precipitates can form in a quantity of 1000 mg/l and more. From the beginning of operation till 01.01.97 more than 27 mln.m³ of sewage was disposed of at "Kama-2". Despite the fact that during that period along with sewage more than 1900 ton of suspended particles were introduced in close-to-face zone. The enlarged well is still being operated with average flow rate of 4 th.m³/day. For comparison, maximum quantity of mechanical admixtures injected with sewage in usual wells during the whole period of operation did not exceed 50-70 ton, and that occurred only in cases when the well developed highly permeable layer having widely branched network of natural fractures /6/. Hence one can conclude that taking into account the influence of suspended particles on the operation of injection wells, one enlarged well can replace 10 and more usual wells.

On 01.01.97 at "Kama-1" object 1,5 mln.m³ of sewage was disposed of. It must be noted that at "Kama-1" object after conduction of the explosion in the well its acceleration characteristics increased more than 20 times, and that was explained by high grade of inhomogeneity of the working horizon. This fact is a sufficient confirmation of the validity of above-stated theoretical considerations on high probability of development of zones with high permeability in inhomogeneous layer by an enlarged well/5,6/.

Sewage disposed of at that object has high toxicity and is not yielded to known methods of purification. Besides, this sewage differ by very high content of suspended particles which in separate periods reaches several hundreds and thousands mg/l, in this case those particles are presented by resin matters possessing absolute mud injection ability referring to layer-collector. Disposal of this sewage through usual wells is practically fully excluded.

Table 1 gives the comparison of technological parameters of new and traditional technology underground disposal of sewage.

In accordance with developed regulations for object operation, technological process for underground disposal of industrial sewage through enlarged wells are being constantly controlled by geophysical measurements conducted in observation wells drilled both to working horizon and to above-lying aquifers.

Control over the movement of the front of sewage along the working horizon is carried out through four observation wells: HB-1, HB-2, HB-3, HB-4 drilled to the working horizon and located at different distances from H-1 injection well, see fig.1. Control over the state of a buffer aquifer is carried out through piezometric HP-1 well. Control over water-bearing horizons containing fresh water and bedding above main water confining layer is carried out through observation wells with depth of 100 m.

In all observation wells current state of the level (pressure) is controlled, stratal water is carried out with subsequent analysis. Besides, gamma-logging, thermometry and resistometry is carried out regularly in observation wells. Results of long-term observations showed that penetration of disposed sewage beyond the boundaries of the working horizon was not registered, and contour of sewage front in the working horizon was in the limits of designed boundaries.

When injected sewage filtrate through explosion zone of a part of radioactive products gets into injected liquid. Source of radioactivity, in this case, represents the radioactive melt of mountain rocks at the bottom cavities and contaminated fragments of mountain rocks in the chimney. Injection of the sewage with a rate of about 4000 m³/hour, results in their partial contamination as a result of desorption from rock surface of rocks and leaching from the melt rock. The contamination of injected sewage in a central zone of explosion is caused, basically, by three radionuclides: 90Sr, 137Cs, and 3H with corre sponding volumetric activity, of about: 7,8x105 bk/l., 1,62x105 bk/l., 1,4x105 bk/l.

However, resulted from dilution of new injected portions and mixing with stratal waters, and also as a result of nuclide adsorption on the surface of rock skeleton sharp decrease is observed as far as radioactive components are concerned in connection with the distance from the enlarged well, that witnessed on safety of utilization of enlarged wells for underground disposal of industrial sewage.

The visual demonstration of the dynamics of passing of sewage front, contaminated by radionuclides, through one of the observation of wells at "Kama-2" object gives the chart, indicated on fig. 2 (on a well HB-3, distant 1000 m from the injection well). More complete information on the character of gamma-activity change, owing to migration, at "Kama-2" object is given on the chart, presented on fig. 3.

On this chart (fig. 3a) change of gamma-activity depending on distance from injection of a well H-1, on various times of operation of object is submitted.

As can be seen from the chart the highest gamma-activity value (about 5000 mkR/hour) will be observed in H-1 injection well, i.e. in the well where an underground nuclear explosion was conducted. However, gamma-activity, registrated in observation wells, is sharply reduced with distance from the source radioactivity.

On the top chart given on fig. 3b in logarithmic scale maximum values of gamma-activity are presented which were registered in the injection wells in any of interlayers of the working horizon for observation period.

The mathematical processing of this data has allowed to establish the law of change of maximum values for gamma-activity depending on distance from the injection of a well, which has the form.

J = 42600 e^-0,0068R (mkR/hour)

(1)

So, using the received law it is possible, for example, to define distance from the injection well, at which gamma-activity reaches background values for rocks, which on the average constitutes the order of 5 mkR/hour. Using equality (1) we shall receive R = 1330 m.

It is interesting to note, that "Kama-2" object, as well as the similar "Kama-1" object (put in to operation later) can be considered as a field model for study of radionuclides migration from underground repository for solid radioactive wastes. In this case melt rocks at the bottom of the cavitys and contaminated fragments of rocks in the chimney can be considered as a repository of solid wastes, and injected liquid, simulates possible movement of stratal liquid through repository, only with greater speed, i.e. the process of migration of radionuclides is simulated with time.

It is necessary to note, that the above-stated data, in our opinion, can be used while forecasting most serious emergency situations when designed underground repository for solid radioactive wastes is unsealed, taking into account that the natural speed of underground waters movement several orders lower, than in this case, besides sorption properties of carbonaceous rocks differ by lower values.

Summarizing the results of research and experimental activities stated above one can make the following conclusions: