CA2185259C - Apparatus and method for subterranean injection of slurried wastes - Google Patents

Abstract

The invention is an apparatus and method for injection of wastes in slurry form within an underground formation. The method consists of: identifying a suitable target stratum;
calculating the approximate total available storage volume of the target strata; injecting a slurry of waste materials into a well in a series of injection episodes separated by interinjection periods; measuring the well bottom pressure of the slurried wastes during each injection episode and interinjection period; and terminating the injection process when the target strata is saturated with slurried wastes. An injection episode is terminated when the well bottom pressure begins to climb substantially above the steady state level, and an interinjection period is terminated when the well-bottom pressure drops below a fixed level. Surface uplift indicators, microseismic measuring and well-bottom pressure within monitoring wells may be used to assess the distribution and movement of embedded solids both during and after the injection process. The apparatus comprises means to produce and inject an appropriate slurry and monitor the entombed slurry during and after the injection process.

Description

~1852~9 APPARATUS AND METHOD FOR SUBTEF;RANEAN INJECTION OF SLURRIED
WASTES
FIELD OF THE INVENTION
The invention relates to an apparatus and process for disposal of wastes in slurried form by deep injection into rock formations at fracturing or overburden pressures.
BACKGROUND OF THE INVENTION
The deep injection of wastes of various types into deeply-buried rock formations is a relatively recent field.
This approach has been suggested for use with radioactive and other types of toxic wastes . For example, U . S . Patent No .
5,310,285 (Northcott) relates to the injection burial of radioactive and other wastes of varying toxicity. The principal advantage of this technique is the potential for stable retention of wastes within a deeply-buried formation over a geological time span.
In general terms, the process involves the preparation of a water-based slurry within surface-based equipment and pumping the slurry into a well that extends relatively deep underground into a receiving stratum. The basic steps in the process include the identification of an appropriate geological site for the injection, preparing an appropriate well, formulation of the slurry, performing the injection operations, and capping the well.

An appropriate target stratum is characterized by pores, fractures or the like. Fractures may also be created within the formation by the injection of wastes under pressure. This approach is taken in U.S. Patent No. 5,314,265 (Perkins et al.). Alternatively, a target strata may be selected that contains existing fractures or pores, as described in U.S.
Patent 5,489,740 (Fletcher). As well, it is desirable that the target zone be depleted of hydrocarbons or other potentially valuable products.
Selection of an appropriate permeable injection stratum leads to rapid bleed-off of fluids, so that the presence of a carrying agent is eliminated and the solids cannot travel far .
Furthermore, once high pressure injection ceases, the solids become permanently entombed within the target stratum by the imposition of the great weight of the overlying strata. The choice of a site with a high permeability horizontal flow system is intended to direct noxious or toxic carrier liquids or leachate to flow laterally, in order that they will not mix with potable groundwaters. The choice of a target stratum with adequate volume, for example a depleted oil reservoir, assures that sufficient pore volume exists to accommodate injected fluids without a regional increase in pore pressure.
The choice of a sedimentary environment which is overall relatively rich in clays means that the leachates which are gradually developed from the solid wastes are rendered more innocuous through cation exchange and adsorption of organic 2»5259 molecules. As an example, a site with multiple low permeability clay-rich beds overlying the target stratum will absorb substantially all noxious ions and dissolved species before long-term contact with shallower groundwater takes place.
Also, the great depth of the burial results in long flow paths for leachates, slow groundwater velocities and a relatively high cumulative exposure to absorptive minerals.
A persistent problem faced by this method is the potential for the eventual migration of the wastes out of the target area and into an aquifer or other undesired destination. This danger may be minimized in part through the selection of an appropriate geological formation to serve as the target for the wastes. However, it is still desirable that the spread of wastes underground be monitored, ideally through the use of monitoring wells and surface monitoring techniques conducted during and after the injection process.
As well, in light of the costs of disposing of wastes by this method and the stringent regulatory environment surrounding the disposal of toxic wastes, it is desirable that monitoring of several additional variables be conducted. This serves to optimize the slurry mixture, injection pressure and rate, and total injection amount. Appropriate monitoring permits an efficient injection process with an optimum amount of slurry being injected. In this way, neither an excessive amount will be injected that might spread beyond the target zone, while generally fully saturating the target zone to make efficient use of the well.
various types of wastes are suitable for disposal by subsurface injection. Potential candidates are wastes that do not react with the target strata, can be readily granulated and can form a slurry suspension in turbulent flow. Wastes that may be disposed of by this means include various substances that are particularly difficult to dispose of in the conventional waste stream, including:
- oily sand from petroleum industry operations, as well as waste drilling fluids and drill chips from well drilling operations and oily slop and sand and residues from tank-bottom clean-outs;
- soil contaminated with toxic materials such as PCB, heavy metals, cyanide compounds, hydrocarbons, etc.;
- dredging wastes;
- municipal sewage sludge from which the organic wastes have been largely decomposed;
- waste plastics, glass, and other solid materials;
- fly ash, clinker or other residue from combustion of wood, coal or municipal wastes;
- flue gas desulphurization sludges as well as recaptured particulates from smoke or emission abatement processes, whether in solid or aqueous suspension form;
- high solids content sludges and residues from petroleum refining, including high ash content coke, heavy oil residues and removed solids.
Apparatus for the carrying out of the slurry injection process must meet several design criteria in order to dispose of a substantial volume of wastes at a high rate:
- injection of slurry at a surface pressure between 6-15 MPa;
- slurry injection rate of between 1.5 and 2.0 m3/min. and 1000 m /day, with injection being carried out for 12-14 hours /day;
l0 - slurry composition with a granular solids content between 15-35~ and real-time waste concentration and slurry density control to maintain density between 1100 and 1500 kg/m3;
- process 200-300 m3/day of granular wastes;
- enhance slurry mobility with waste materials having about 10~ by volume hydrocarbon content;
- capability to accept slop or sand as waste material.
As well, the apparatus should be capable of operating on a generally continuous basis, and comprise an integrated system that is adapted to receive wastes, convert the wastes into an appropriate slurry, and discharge the slurry under pressure into a disposal well.
The operating parameters require equipment capable of injecting a relatively granular, highly viscous slurry at high rates and pressures. Preferably, the slurry formation and injection apparatus should provide the following:

a) relative ease of handling of waste material;
b) screening of granular waste material on a continuous basis;
c) a real-time monitoring apparatus to monitor and record injection parameters;
d) variable speed controls linked to the monitoring apparatus to control the various slurry-forming components and maintain consistent slurry quality and delivery rate;
e) relatively rapid set-up and disassembly of the system;
f) slurry formation equipment capable of shearing highly viscous material to increase slurry mobility and injectivity, maintaining slurry consistency within a relatively small range, and being capable of handling relatively large amounts of waste material, in the range of at least 100 rrt~/day.
Monitoring of conditions within the target stratum serves two functions . First, it insures that the inj ection procedure is optimized for maximal injection speed and overall waste injection volume. Second, it provides evidence to regulatory agencies and other outside bodies that the injection process is being properly implemented and that the wastes are being confined within the target stratum. These goals may be furthered by monitoring and recording several variables in addition to those outlined above. In particular, slurry density, injection pressure, volume and composition should be monitored and recorded at all times. Alterations in large-scale permeability within the target stratum, excessive pressure build-up, abnormal fracture pressure, too-rapid pressure decay or other anomalous reservoir responses can be identified and analyzed to decide if these present problems for the continuation of the injection process in a particular well.
ST7I~1ARY OF THE INVENTION
In light of the objectives outlined above, the present invention comprises an apparatus and method for injection of wastes within an underground formation by means of pumping pressurized wastes in slurry form into a suitable underground formation. Preferably, the method includes regular and periodic monitoring of the spread of the wastes within the subsurface formation during the injection and after the injection concludes to ensure that the wastes do not travel outside the target zone.
The method comprises in its broadest aspect the following steps:
a) identifying a generally permeable and porous target stratum, overlain by a layer of relatively low permeability strata;
b) calculating the approximate total available storage volume of the target strata, based on the approximate average thickness and area of the stratum, the average porosity of the stratum and the mechanical compressibility of the formation;
c) preparing an at least partly cased well extending from the ground surface into the target stratum;
d) positioning a pressure gauge at the base of the well for measuring formation pressure within the well;
e) perforating the well casing where the well passes through the target stratum;
f) injecting a slurry of waste materials in particulate form suspended in a carrier liquid into the well in a series of injection episodes separated by interinjection periods.
with the injection pressure being greater than or equal to the fracture or overburden pressure;
g) measuring the well bottom pressure of the slurried wastes during each injection episode and interinjection period;
h) terminating the injection process when the target strata is generally fully saturated with slurried wastes, as determined by the volume of wastes injected and the calculated available storage volume.
The method is further characterized in that the pressure and flow rate of slurry during each injection episode is adjusted to maintain a generally steady pressure at the well bottom, as measured by the pressure gauge. An injection episode is terminated when the well bottom pressure begins to climb substantially above the steady state level, indicating blockage within the target stratum. An interinjection period is terminated when the well-bottom pressure drops below a fixed level, indicating sufficient dispersal of the injected t fluid within the stratum.
An additional step that may be taken to determine the duration of the injection episodes comprises the use of a surface uplift indicator, for example one or more tiltmeters, positioned in the region around the injection well. A
specific injection episode is terminated upon measurement of a fixed amount of uplift, with the duration of an interinjection period being determined by assessing the approximate cessation of further surface deformation. The uplift indicators may also be used to determine the approximate spread of the injected solids within the target strata.
An additional indicator consists of measuring well-bottom pressure within monitoring wells in the region about the target strata.
Mathematical analysis of the pressure and surface deformation data may be conducted to determine the orientation and distribution of the injected slurry. Microseismographic data may also be used for this purpose. This analysis assists in evaluating containment of the material within the disposal formation.
In one aspect of the invention, appropriate target strata and overburden are identified, with the target strata having a minimum average thickness of approximately 4 meters, a minimum average transmissivity of approximately 0.5 Darcy-meters and a minimum porosity of about 15~ in those regions .~ ~f 2185259 that have an average permeability above about 100 milliDarcy.
The overburden confining strata have a minimum thickness of about 10 meters with a maximum permeability of about 10 milliDarcy, and will preferably be relatively rich in clays.
In a further aspect, microseismic disturbances are measured at several locations in the region of the injection well, to assess the geographical extent and distribution of the waste solids emplacement zone within the target stratum and the extent of growth of the solids emplacement zone.
In a further aspect of the invention, post-injection monitoring of the target stratum is conducted to ensure that the waste solids entombed therein are not migrating from the target stratum. The post-injection monitoring comprises the measurement of surface uplift and microseismic disturbances in the region of the inj ection well and determining therefrom the size (thickness and horizontal spread) of the solids emplacement zone. These measurements are monitored and recorded on a periodic basis to assess the extent of any spread of the solids emplacement zone.
The apparatus consists in its broadest aspect the combination of particle sizing means to remove oversize particles, a mixing-averaging means for the combining of particles with water to create a slurry, and a pump for the delivery of the slurry under pressure to the wellhead. The particle sizing means incorporates a reciprocally-driven multilevel screen deck that removes over-sized material (i.e.

>0.25-lcm) and foreign objects, and a means to direct a high-pressure water spray over the waste being sized. The sized particles are received in a receptacle which houses a rotatably-driven auger to mix together the sized particles and water to generate a slurry. The receptacle preferably houses dual rotatably-driven augers. The first auger is a mixing auger positioned in the pathway of material being introduced into the receptacle, and the second auger is positioned at the bottom of the receptacle. Pump means are linked to the receptacle to pressurize the slurry for delivery to the well.
The use of dual augers serves to shear highly viscous slurry material, thereby increasing slurry mobility and injectivity. As well, the dual augers permit delivery of a consistent slurry in terms of solids content and consistency, and permit efficient movement of a relatively large volume of granular waste material.
waste receiving means are preferably provided to receive the wastes. These means may comprise a hopper linked by conveyor to the particle sizing means.
The apparatus may further include control means for controlling the system and data processing and storage means linked to the apparatus to monitor and control its operation and to monitor the injection process. A real-time monitoring system may be linked to the control means, comprising slurry density measuring means, pump/surface injection pressure measuring means; means to measure slurry injection rates and 2185~~9 water input into the system, and a data logger. Preferably, the various components are driven by variable speed hydraulic motors controlled by the control means to provide consistent slurry quality and delivery rate and pressure.
The apparatus may further include a computer adapted to receive information from pressure gauges positioned at the bottom and surface of the inj ection well and within one or more monitoring wells. Preferably, information is also received from one or more surface uplift indicators and seismic geophones in the region around the injection well.
The computer is programmed to receive the data and assess the approximate localization of the wastes within the injection strata.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a cross-sectional schematic view of an underground formation with an injection well therein, with monitoring and injection equipment in position;
Figure 2 is a schematic view of the slurry-forming apparatus according to the present invention;
Figure 3 is a perspective view of the components of the apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

-~- 2185259 The first stage of the process entails the identification of a suitable inj ection site and target stratum, identified in Figure 1 as reference 2, and to determine the extent of the site. The preferred site and target stratum will have the following characteristics:

a) The site will be free of significant quantities of hydrocarbons and other valuable minerals that could become the target for exploitation in the foreseeable future.
b) The target stratum will have a minimum thickness of 4 meters and be at least 0.5 x 2.0 km in extent, or be connected hydraulically to permeable strata that have these dimensions.
c) The minimum transmissivity of the stratum will be 0.2- 0.5 Darcy-meters (average permeability in Darcy units multiplied by the stratum thickness in metres).
d) The direction of groundwater flow in the region around the stratum will be generally horizontal, i.e. , deviating from the horizontal by no more than about 2.5 degrees.
e) The target stratum will comprise granular, poorly-cemented or uncemented sandstone or highly fractured porous rock. The rock will have no significant large-scale tensile fracture strength (tensile resistance to hydraulic fracturing) and a compressibility of at least 1x10!6 kPa~.
f) The rock in the stratum will not react chemically with the solid or liquid phases of the slurry to release any free gases or new noxious compounds, apart from the ordinary slow dissolution of minerals which takes place in aqueous systems.
g) The stratum will have a minimum porosity of about 15~ in the portions that have a permeability above 100 Md (milliDarcy).
h) The stratum will have a generally horizontal orientation, with an overall declination of no more than about 2.5 degrees per kilometre . The stratum wlll noz ~e 1m ~C1 ~~~ ~~u wr faults or other geological features that could give direct or easy access from the stratum to surface groundwater.
i) The stratum will be overlain by at least 10 meters of low permeability overburden strata with a maximum permeability of 10 Md.
Additional requirements are imposed for the selection of target strata where it is desired to dispose of wastes that will produce mildly or substantially toxic liquids ( as opposed to merely salty or noxious ) , whether such liquids comprise the injection fluid itself or leachate resulting from interaction between the solid wastes and the carrier liquid. These additional requirements are, at a minimum:
a) The well depth will be a minimum of 250 m in regions of modest surface elevational differences (20 m or less) and 500 m in hilly terrain (20-200 m).
b) The well will be isolated from all shallow groundwater sources by laterally continuous overburden strata that extend a minimum of about 10 km in all directions . These barrier strata will have a combined thickness of 50 m, counting only those strata that are at least 5 m thick and shall have an aggregate transmissivity to water no greater than 0.02 Darcy-meters.
c) There will be at least one "security zone" consisting of a permeable layer above the target zone to bleed off any liquid that might migrate upwards. The security zone should have a transmissivity of at least 1.0 Darcy-meters and be dominated by horizontal groundwater flow, unconnected to shallow potable water sources.
d) The groundwater in the target stratum, security zone and all other affected strata must not be a source of potable water, and must be shown to be hydraulically isolated from regional potable drinking water (no local faults through jointed rocks). This isolation can be demonstrated isotopically or through groundwater or lithostratigraphic studies.
e) The overburden should include a sedimentary sequence that includes minerals such as clays or zeolites which have the capacity to absorb wastes such as heavy metals and organics.
A waste-bearing slurry is prepared, with solid wastes being suspended in the slurry in particulate form. The maximum particle size of suspended wastes is about 5 mm with no more than 15s of the particles being larger than 2.5 mm.
Slurry design parameters include:
- the amount of non-dissolved solid material in the slurry;
- the liquid phase viscosity (which will affect the injection rate and underground pressure bleed-off);
- colloidal material content (clay and polymers);
- non-aqueous liquids content (eg. oils and other immiscible liquids);
- content of cementitious or other viscosifying agents (portland cement, lime, etc.); and - additives used to enhance or alter the rheology of the slurry, such as polymers, thickeners, gels or emulsifiers.
Cementitious of other visclosifying agents are added to slurries that include wastes of intermediate or high toxicity, such as PCB-.w.

.-- 2185259 contaminated soil, radioactive wastes, and heavy metals or arsenic compounds. The use of cementitious or other viscosifying agents reduces the permeability of solid waste materials within the target zone. Thus, after the carrier liquid bleeds off, a solid waste body results that has a permeability substantially lower than that of the surrounding rock, with a consequent hydraulic isolation of the target waste body and a reduced leachate generation rate.
Cementitious or viscosifying agents may include:
- finely ground gypsum, limestone or lime - finely ground FGD (flue gas desulphurization) sludge, which serves as permeability blocking agent - ground shale, clay or other finely divided material - synthetic non-biodegradable polymer agents - portland cement or other commercial cements or pozzolanic cementitious agent - fly ash or finely ground combustion clinker The slurry mixture may be tested prior to injection for the reduction in its permeability. The tests may comprise quantitative analysis using high-pressure uniaxial and triaxial compressibility cells with creep testing and fluid flow rate testing methods. To test the ultimate permeability of a particular cementitious slurry, a sample of the mixed slurry is placed in a test chamber and subjected to the overburden stress, pore pressure and temperature it will experience at depth. Compaction is allowed to occur by making A

provision for the drainage of expelled pore liquid, simulating 17 (a) -..
i.

zls~z~9 the densification process at depth. These conditions are maintained on the test sample for a period of no less than two weeks , at which time an axial permeability test is carried out in the same manner.
The axial permeability test consists of determining the flow velocity through the test sample when a water pressure difference is imposed between the top and bottom of the compacted cementitious solid material. To be effective as a means of isolating the wastes from surrounding groundwater flow, the permeability should be no more than 1~ of the average permeability of the target strata.
The slurry is also formulated to ensure rapid bleed-off of pressure during injection, so that fracturing does not propagate far, either vertically or horizontally.
The typical injection well 68 will be similar to a conventional oil well, with a substantially full-length production casing 3 and, where appropriate, a cemented surface casing 4. The well will not deviate from the vertical by substantially more than 45 degrees. The lower portion of the well casing, comprising a length of at least 3 meters, is perforated. An open-bottom injection tubing string 5 is lowered into the hole and there retained with a casing-packer 6 in such a manner that the lower part of the tubing string protrudes below the packer and at least 1 meter above the uppermost perforation.
During installation of the tubing string, a bottom-hole electronic pressure gauge 7 is installed to measure pressure of the fluid within the tubing near the bottom of the hole.
The gauge is submersible, and may comprise a strain gauge, vibrating wire or vibrating strip-type gauge, capable of measuring bottom-hole pressure from 7-70 Mpa. The gauge is installed in a stainless steel saddle 8 welded to the tubing string 0.5 to 3 meters above the top of the packer. The sensor provides an electronic signal to the surface through a multiconductor, high-pressure cable 9 strapped to the outside of the tubing and installed along with it. The gauge serves to measure fluid pressure of the slurry at the bottom of the well during and after the injection. A second electronic pressure sensor or continuous pressure recording device 10 is installed within the annular space between the casing and the tubing at the wellhead.
The slurry may be formed either in advance and trucked to the site in the pumping truck 12, or prepared on site. The density of the slurry is monitored either at the pumping truck exit line 14 or at the exit line from the slurry formation apparatus. Where the slurry is mixed on site, the quantities of waste solids, aqueous carrier, additive solids and waste liquids entering the slurry are monitored.
Prior to slurry injection, it is desirable to conduct pressure fall-off and step-rate injectivity tests within the inj ection well in order to assess the stress state, fracturing pressure and transmissivity of the reservoir. The information obtained from these procedures is used both to optimize the injection protocol and during the slurry injection procedure to determine if anomalies are occurring.
Slurry is injected into the well in a series of one or more injection episodes of between about 3-30 hours each. The injection episodes are separated by interinjection periods of between 5 and 100 hours, depending on the response of the stratum. Each injection episode is initiated by pumping solids-free liquid through the system at a pressure sufficient to initiate fracturing of the target stratum. Typically, the flow rate during this stage will be about 1.5 m3/min. Solids are gradually introduced to the flowing mixture, and the target solids content is built up over 15-20 minutes. At the end of each injection episode, the solids content of the slurry is gradually diminished, the well is flushed with clear liquid and the well is shut in under pressure (i.e., while pumping). The wellhead and well bottom pressures are recorded at this stage and periodically thereafter throughout the interinjection episode.
The injection well and the region surrounding the well are monitored in various respects, in order to achieve several goals. In particular, appropriate monitoring permits optimization of: the total slurry volume to be injected; the injection rate; the slurry density and composition; the duration of the inj ection and inter-inj ection episodes and the total period of use of the injection well. These factors are r'v 2185259 determined as follows:
a) The total slurry volume to be injected for each well or series of wells is a function of the proj ected interstitial volume of the target stratum. This is determined by assessing the size of the target stratum (both area and thickness) and the approximate interstitial volume of the stratum.
b) The target flow rate for the slurry is determined by the speed at which the target stratum is capable of absorbing the slurry while maintaining a steady state well bottom pressure.
Ideally, this pressure is between 115 and 135 of the overburden stress. The well bottom pressure is measured by the pressure gauge 7 positioned at the well bottom.
c) The duration of each injection and inter-injection episode is determined by the rate at which the slurry spreads within the target stratum and the rate at which the pressure at well-bottom dissipates. The spread of slurry within the target stratum is assessed primarily by the magnitude and decay rate of ground surface uplift in the region around the well, as determined by tiltmeters or other surface uplift indicators. Typically, maximum surface uplift during an injection episode will be limited to a maximum of about 5cm or such other value as may be set by a regulatory body . Once this amount of uplift has occurred, the episode is halted until pressure decays to an acceptable level (i.e. less than 80~ of overburden stress) and the deformation rate is small.
d ) The localization of the inj ected solids within the target c; '~ i~ ;~ a '-~
2~g5259 stratum is determined by microseismic surveillance, through the use of seismic geophones, and measurement of surface uplift using tiltmeters or the like. The localization of the region of uplift provides an indication of the lateral spread of the embedment zone. Microseismic surveillance assesses the horizontal and vertical positioning of the zone.
Additionally, pressure characteristics within monitoring wells in the region around the inj ection well may be taken into account. An injection episode should be terminated if the well-bottom pressure within a remote monitoring well (more than 50m distance) climbs by about 25~ of its original pressure (or such other value as may be identified as critical).
As well, any particular information that is available regarding the structure and seismic characteristics of the target stratum may be taken into account. For example, unexpected microseismic activity or anomalous pressure response in an adjacent monitoring well can result in a modification of one or more the parameters set out above.
An example of a typical injection protocol developed for a site in East-Central Alberta is:
Tartlet Stratum Description: 14 m. thick, 30~ porosity depleted sandstone reservoir; compressibility of 1x10 Kpa or higher; flat-lying ( horizontal ) of great lateral extent ( > 1 km in all directions).
Description of overlying strata: directly overlain by -.. . , 2185259 100 m alternating shales and clayey silts; permeability less than lOmD, except for several thin stringers (1-3 m) of permeability >100 Md, for lateral bleed-off of any vertically migrating fluid; from 100 m above the target to 250 m above the target, a continuous bed of ductile shale (horizontal) of extremely low permeability.
Slurry Composition: The carrier phase is waste water (weak brine) produced along with oil from an adjacent oil field (70~-80~ of slurry volume). The solid waste is fine-grained sand with a small fraction of clay (<1-2~) contaminated with heavy oil (15-30~ of slurry volume). Also, the slurry may include 0-25~ of "slops", i.e., finely ground surface wastes, including soil, sand or water mixed with spilled oil.
In-iection Pressure: Measured at hole bottom, no greater than 140 of overburden stress.
Infection Rate: from 1.1 to 2.0 m3/min. of slurry.
Total Slurry Volume: Max 800 mj in a 24 hour period.
Total for well - 100,000 m'of 20~ solids content for a total of 20,000 m of sand.
Average Infection Duration: 10 hours Average Interin~ection Period: 14 hours Maximum Surface Uplift: less than 1 mm for each episode.
Monitorinct strategy: Four pressure monitoring wells in a square, each well being 150-300 m from the inj ections well; 10 tilt meters arranged in a first circle of 5 placed at 150 m '~ 2185259 radius around the injection well and a second circle of 5 at 300 m radius from the injection well. Annular casing pressure, tubing wellhead pressure and tubing well bottom pressure recorded during injection and interinjection episodes. Injection volumes, rates and solids contents measured and recorded In most applications, additional variables should be monitored and recorded, in particular slurry density, pressure, volume and composition. The pressure data from the injection and monitoring wells are used, together with step-rate injection and pressure fall-off test data, to evaluate the waste emplacement process.
Alterations in large-scale permeability, excessive pressure build-up, abnormal fracture pressure, too-rapid pressure decay or other anomalous reservoir responses are identified and analyzed to decide if these present problems for the continuation of the injection process in a particular well. For example, if the monitoring wells display sudden pressure responses, this would indicate that a discrete fracture plane is interacting with the remote wells, thereby suggesting that the fracture bleed-off is being impaired by permeability blockage. If this is the case, or if other anomalous responses are noted, the slurry design and the injection strategy are altered to rectify the problem and remain within the realm of rapid bleed-off and near-wellbore solids emplacement.
Injection procedures may be adjusted appropriately in the event that well-bottom pressure is decaying too slowly, if it appears that solids are being transported out of the target stratum, if the monitoring wells show anomalous pressure responses, or if other monitoring reveals substantive formation containment impairment. For example, slow strain relaxation and pressure decay may be due to excessive fines in the slurry, too large a volume injected within each episode or too short an interval between injection episodes.
The response of the reservoir stratum and overlying rock to the slurry injection may be assessed by way of the surface deformation data, in combination with the previously-determined capacity of the reservoir. The reservoir response may be determined from this data as follows:
a) The tiltmeter response data over the time period of interest (i.e. 1 hr. to several days) is examined to ensure against anomalous noise signals in the data base.
b) The magnitude and direction of the surface tilt responses provide input to a computer programmed to analyze the tiltmeter data.
c) The analysis provides an estimate of the size and shape of the zone of solids emplaced at depth over the time period analyzed.
Confirmation that the injection process is proceeding properly is obtained by assuring that there is a rough balance between the solids volume input and the volume of deformation, by comparing known input to the results of the mathematical analysis.
The surface uplift data allow discrimination between vertical and horizontal fracture orientations by virtue of the magnitude and direction of the tilt vectors from an array of 10-20 tiltmeters positioned around the injection well. This indicates whether vertical or horizontal material transport away from the wellbore may be occurring. In general, the tilt response for long-term injection wells should be dominated by horizontal fracturing components. The tilt data can be analyzed in terms of total deformation to give limits on the extent of the deformation in the reservoir, and by this means the approximate radial extent of the emplacement zone can be assessed. Also, the tilt or deformation data can be used directly to demonstrate that the ground surface deformations are small and meet limits which might be set by regulatory guidelines.
The time-dependent decay of surface tilt changes and internal pressures provides direct evidence of the speed by which the reservoir and the overlying rocks are responding to the volumetric and pressure changes induced by the injection activity. If deformations continue slowly for many days after an injection episode, combined with a slow pressure decay rate, it is proof that the reservoir is approaching capacity, w-~ 2185259 that permeability has become blocked, or that injectate has migrated to a zone of low fluid transmissivity. Conversely, rapid decay and cessation of deformation is evidence that the reservoir is responding as expected with efficient bleed-off and solids localization near the wellbore. These measures over time are used directly to adjust the slurry design and the injection strategy to achieve the best possible reservoir response to the injection. Mathematical analysis of the pressure and tilt data allows for reconstruction of the size and distribution of the injected material.
The method may include microseismic monitoring of the surrounding region to assess the injection process. Such monitoring involves detecting and analyzing small seismic disturbances associated with rock deformation and stress changes that accompanies the slurry injection. Microseismic monitoring is used in conjunction with the surface deformation and uplift data to determine the approximate dimensions horizontally and vertically of the solids emplacement zone.
The locations of microseismic events are plotted three-dimensionally over time, and resulting identification of the microseismic emission field identifies the size and growth rate of the solids emplacement zone. If large amounts of microseismic activity are observed high above or far away from the perforation locations in the well, the nature of the signals is analyzed along with the surface uplift response to the injection, to ensure that solids are not migrating out of ~I85259 the injection zone. If the microseismic emissions continue beyond the time of active injection by several hours or days, this is taken as evidence that pressures are not decaying sufficiently rapidly or have entered a zone of lower permeability. The data from microseismic monitoring are combined with other measures (tilt, volume, rate, pressures) to permit the inj ection process to be controlled and optimized continuously.
The monitoring data are analyzed to carry out the operations described above with a computer linked to the monitoring instruments described above and programmed to perform the following operations:
Data pre-processing: noise reduction/filtering Calculate tilt vector Assess: data waveform displacement field Quality: P + S - wave time Picking/seismic vector tracing v Analyses of tilt data Analyses of seismic data (output) ~ (output) Reconstruct fracture Determine source locations, geometry/deformation source size, stress state parameters L
Assess reservoir response to injection operations Assess containment of injected material in formation These operations permit the rapid assessment of events within the formation, and permit the dimensions of the solids containment area to be evaluated. This in turn permits the user to demonstrate that the solids are being appropriately contained within the target stratum.
Post-injection monitoring is carried out to ensure that the solid wastes are generally contained within the target stratum. The subsurface pressure (measured in both in the injection and monitoring wells), surface deformation and microseismic monitoring described above is carried out subsequent to the inj ection, typically for a period of several days. If the site has been properly selected and the injection properly carried out, the post-injection monitoring should disclose stable underground conditions. If surface or subsurface instability continues after the injection terminates (allowing for a period of approximately one week for stabilization to be achieved), this is evidence that the solids are potentially migrating out of the target zone or that near-wellbore placement of the injected material is not occurring.
Surface deformation and microseismic analysis as described above is also deployed in the post-injection period to determine on a periodic basis the positioning of the solids emplacement zone, to ensure that this zone is not expanding beyond set limits and is not potentially communicating with potable water.

Referring to Figures 2 and 3, the slurry formation and injection apparatus comprises in general terms a feed hopper 30, mixing-averaging apparatus 32 and injection pump apparatus 34. A conveyor 36 transports waste material from the hopper to the mixing-averaging apparatus and comprises a rotatably-driven auger 37 housed within an elongate chamber 38. A water supply tank 39, linked by pipe 40 to the mixing-averaging apparatus, provides a steady high-pressure (approx. 200 psi) source of water for the creation of the slurry. A pipe 64 transports the slurry from the mixing-averaging apparatus to the pump 34.
The feed hopper 30 comprises waste-receiving means and is utilized for wastes that consist of oil or sludge-bearing sand, or the like. For certain other, more fluid types of wastes, the hopper may be dispensed with and the wastes deposited directly into the mixing-averaging apparatus. The hopper is designed to receive a load of between 8 and 20 cubic meters of sand.
The mixing-averaging apparatus comprises a particle sizing means to screen out oversized particles, consisting of a reciprocally-driven multilevel screen deck 50 onto which wastes are deposited from the conveyor 36. The individual screens within the deck are adjustable and removable to optimize slurry composition for particular injection conditions. A water sprayer 52 is positioned to direct a high-pressure stream of water at the wastes as they exit the h , ,~ 2185259 conveyor 36. The sprayers are linked to the pipe 40. The screen deck is comprised of three levels of screens, each having a variable matrix size. Waste material is dumped onto the uppermost screen deck 58 either directly or from the conveyor 36. The action of the spray jet and the shaking of the screens serves to remove particles having a size greater than 0.25 to 1 cm. and foreign objects in the waste stream.
These oversized particles are either crushed by a stand-alone crusher 60, to be fed back into the waste stream, or are collected and disposed of by other means, not shown. The screened wastes fall from the screen deck into a second auger-driven conveyor 65 which transports the particulate mixture frm the scren deck 50 into the open top of a slurry averaging and mixing tank 61 that supports therein dual rotatably driven mixing screw augers 62 and 63. Additives and agents may be added directly into the tank 61, if required.
A pipe 64 leads from the base of the slurry averaging tank into the booster pump apparatus 34, which pressurizes the slurry and discharges it under pressure through a discharge pipe 66 into the well 68.
The various components of the system are driven by conventional variable speed hydraulic motors 70. These in turn are linked to a control means 72 which permits control over the inputs into the slurry-production means and over the slurry design. The control means receives input from a real-time monitoring system that monitors, records and visually ~. 2185259 displays the injection parameters of slurry density, injection rate, surface 31 (a) A

injection pressures, injected volumes and slurry solids concentration. The monitoring system consists of .
- slurry density measuring means 80;
- pump pressure measuring means 82;
- means 84 to measure water input into the system;
- means 86 to measure injection rates;
- a control data logger 88 linked to all of the measuring means to record and store the data in real time. A digital display 90 is provided in the data logger. The monitoring system is also linked to and receives data from the pressure gauges 7 and 10 at the well bottom and surface.
The control means is adapted to maintain an even slurry density and delivery rate and pressure. The means by which this is achieved comprise generally conventional feedback means.
The apparatus further includes a computer operatively linked to the surface uplift indicators and, optionally, to the microseismographs described above, and programmed to assess the localization and movement of the solids embedment zone in the manner described above.
The present invention has been described by way of a specific embodiment thereof. It will, howver, be understood by those skilled in the art to which this invention pertains that numerous departures from and variations to this invention may be made, while remaining within the spirit and scope of the invention, as defined in the appended claims.

Claims (17)

1. A method for disposing of wastes by injection within a geological stratum, comprising the steps of:
a) identifying an appropriate generally permeable and porous target stratum, said target stratum having a minimum average transmissivity of about 0.5 Darcy-meters, a minimum porosity of about 15% in those regions that have an average permeability above about 100 millidarcy and a minimum .6 compressibility of about 1 x 10 -6 Kpa -1 and being overlain by a layer of relatively low permeability strata;
b) calculating the approximate total available storage volume of the target strata, based on the approximate average thickness and area of the stratum and the average porosity of the stratum;
c) preparing an at least partly cased injection.well extending from the ground surface into said target stratum;
d) positioning a pressure gauge within said injection well in the region of the base of said well for measuring liquid pressure within said well;
e) perforating the casing of said injection well where the well traverses said target stratum;
f) injecting a pressurized slurry comprising waste materials in particulate form suspended in a carrier liquid into said injection well in a series of discrete injection episodes separated by interinjection episodes at an injection pressure equal to or greater than the fracture or overburden pressure;
g) measuring the well bottom pressure of the slurried wastes with said pressure gauge on a continuous basis during each injection and interinjection episode with said pressure gauge;
h) terminating said injection when the target strata is generally fully saturated with slurried wastes, as determined by the amount of wastes injected and the calculated available storage volume;
said method being further characterized in that the pressure and flow rate of slurry injection during each injection episode is adjusted to maintain a steady state pressure level at the well bottom, as measured by the pressure gauge, each injection episode terminating when the well bottom pressure climbs substantially above said steady state level and each interinjection period terminating when said pressure drops below said steady state level.

2. A method as in claim 1, comprising the further step of placing one or more surface uplift indicators on the ground surface in the region around said injection well surface.

3. A method as in claim 2, wherein said surface uplift indicators are employed to assess the approximate horizontal and vertical spread of the waste solids emplacement zone within said target stratum and the extent of growth of the solids emplacement zone.

4. A method as in claim 2, wherein said surface uplift indicators are employed to determine the duration of said injection episodes, each of said injection episodes being terminated upon measurement of a fixed amount of uplift, with the duration of said interinjection episodes being determined by assessing the time of approximate cessation of further surface uplift.

5. A method as in claim 2 wherein said surface uplift indicators comprise tiltmeters.

6. A method as in claim 1, wherein the duration of said injection and interinjection episodes is further determined thorough the use of monitoring wells located at least 50 meters from said injection well, each of said monitoring wells having a pressure gauge in the region of the bottom thereof, said injection episodes being terminated when the pressure as measured in said monitoring wells climbs by more than about 25% of the original pressure within said wells.

7. A method as in claim 1, wherein said target stratum has a minimum average thickness of approximately 4 meters.

8. A method as in claim 7, wherein said target stratum is overlain by overburden strata having a minimum thickness of about 10 meters with a maximum permeability of about 10 milliDarcy.

9. A method as in claim 1, comprising the further step of positioning a second pressure gauge within said well at the region of the wellhead, and monitoring and recording the pressure of the injected slurry therewith on a continuous basis during said injection and interinjection episodes.

10. A method as on claim 8, wherein said overburden strata include clays or zeolites for the absorbtion of toxic organic and heavy metal wastes.

11. A method as in claim 1, comprising the further step of measuring microseismic disturbances at several locations in the region of said injection well, and assessing thereby the approximate horizontal and vertical spread of the waste solids emplacement zone within said target stratum and the extent of growth of the solids emplacement zone.

12. A method as in claim 1, comprising the further step of incorporating within said slurry a cementitious or other viscosifying agent.

13. A method as in claim 1, wherein the duration of each of said injection episodes is between 3 and 30 hours.

14. A method as in claim 1, wherein each injection episode commences with the injection of a carrier liquid clear of wastes, with the concentration of suspended wastes increasing therein over the initial 15-30 minutes of said injection episode to reach the target concentration of said suspended wastes, with said injection episode concluding with the injection of additional clear carrier liquid, with the injection well then being shut in under full injection pressure.

15. A method as in claim 1, comprising the further step of monitoring of the target stratum to assess migration of the waste solids entombed therein from the target stratum, said monitoring comprising the measurement of surface uplift in the region of the injection well, monitoring and recording said measurements on a periodic basis, and determining thereby any instability within the solids emplacement zone and the localization of the solids emplacement zone, and comparing the said localization with the localization thereof immediately following the injection process to assess any volume change in the site or movement thereof.

16. A method as in claim 15, comprising the further step of measuring microseismic disturbances in the region about said injection well to assess thereby any instability in the solids emplacement zone and the localization thereof.

17. A method as in claim 15, comprising the further step of post-injection monitoring of well-bottom pressure within the injection well and monitoring wells located around said injection well, each of said monitoring wells having a pressure gauge in the region of the bottom thereof.