METHOD OF APPLICATION OF LOW SALINITY VISCOUS FLUID IN DRILLING SALINE FORMATIONS

Abstract

The present invention addresses to the use of a viscous fluid in continuous pumping to contain the saline dissolution in a homogeneous way (not limited to just one type of evaporite), highlighting the results of seawater with viscosifiers, due to its simplicity, and that does not have the same impact on the logistical chain as a brine supply. The increase in fluid viscosity limits the diffusion of salt into the medium and thus better contains the dissolution of the well walls, while also promoting a laminar flow regime in the annulus of the well, variables desired to achieve the quality of the operation of cementation. The employment of viscous fluid concentrates allows “on the fly” dilutions with the in-line seawater fraction mixture. The technique can also employ solutions of pre-dispersed viscosifying additives which, when added to seawater, result in a substantial increase in the volume of drilling fluid produced, no longer limited by the unit tank capacity. This allows drilling large extensions of salt. The low salinity viscous fluid is used in drilling operations without fluid return to the drilling rig (riserless) in the presence of a predominantly saline formation, aiming at preserving the phase diameter and improve well construction conditions.

Claims

1. A method of application of low salinity viscous fluid in drilling saline formations, characterized in that it comprises the following steps: (a) Preparation of the fluid: (a1) Preparing the Pad Mud for dilution with seawater during pumping; (a2) Pumping the Pad Mud in the form of cleaning plugs with or without dilution with seawater, according to the programmed viscosity, during the previous formation drilling to the saline layer (post-salt); (a3) Checking the alignment of the brine tanks with fluid to feed the surface tanks; (a4) Checking the alignment of the fluid and seawater suction lines, as well as the alignment on the standpipe manifold for the Pad Mud and seawater mixture; (b) Phase drilling: (b1) Drilling with seawater with a flow rate between 1,000 to 1,200 gpm (3.7854 to 4.5425 m.sup.3/min), and pump 80 to 120 bbl (9.539 to 14.309 m.sup.3) of viscous fluid or Pad Mud 3.0 to 6.0 lb/bbl (11.4 to 22.8 kg/m.sup.3) for every 3 drilled sections; (b2) Evaluating cleanliness and adjust plug frequency with drilling parameters; (b3) Proceeding with seawater drilling and displacement of periodic fluid plugs to the top of the saline formation; (c) Pump and Dump: (c1) After identifying the change in the formation of anhydrite to halite, starting the Pump and Dump operation, reducing the seawater flow rate and adding the second pump with the Pad Mud flow rate so that the mixture is equal to the total drilling flow rate and the desired final concentration; (c2) Maintaining the total drilling flow rate, mixing PAD Mud and seawater, so that the final mixture after the standpipe manifold contains at least 1.0 lb/bbl (3.8 kg/m.sup.3) of viscosifier; (c3) After the start of the Pump and Dump, when the first tanks of fluid on the surface run out, immediately starting the preparation (or dilution) of the complement of filling fluid for running the casing down, and for transfers during the withdrawing of the bit; (c4) At the end of the phase drilling, pumping the saturated filling fluid into the bottom to occupy the entire evaporite section; (c5) Withdrawing the string to the top of the anhydrite and pumping filling fluid to fill the remainder of the annulus to the seabed.

2. The method according to claim 1, characterized in that the Pad Mud is prepared at a concentration of 1.0 to 6.0 lbm/bbl (3.8 to 22.8 kg/m.sup.3) of xanthan gum, or guar gum, or carboxy-methyl-cellulose.

3. The method according to claim 2, characterized in that the Pad Mud is prepared in the preferential concentration of 6.0 lbm/bbl (22.8 kg/m.sup.3) of xanthan gum.

4. The method according to claim 1, characterized in that Pad Mud is alternatively prepared in concentrations of xanthan gum greater than 6.0 lbm/bbl (22.8 kg/m.sup.3), provided that the final viscosity allows its pumpability.

5. The method according to claim 4, characterized in that there is the adjustment of the pumping flow rate of the Pad Mud prepared in concentrations of xanthan gum greater than 6.0 lb/bbl (22.8 kg/m.sup.3) and seawater so that the output in the standpipe manifold at the viscosifier final concentration in the mixture is a minimum of 1.0 lb/bbl (3.8 kg/m.sup.3).

6. The method according to claim 1, characterized in that the fluid is prepared with additives such as: filtrate reducer, bactericide, alkalizing agent and salt.

7. The method according to claim 1, characterized in that part of the Pad Mud is stored in surface tanks and another part in reserve tanks, or even brine tanks.

8. The method according to claim 7, characterized in that the Pad Mud is stored prior to the operation with bactericide.

9. The method according to claim 7, characterized in that the Pad Mud is periodically recirculated to the surface tanks, treating the returned fluid with a bactericide and maintaining the pH in the range of 9.0 to 10.5 with an alkalizing agent.

10. The method according to claim 9, characterized in that the bactericide is glutaraldehyde.

11. The method according to claim 1, characterized in that the flow rate ratios between the seawater pump and the Pad Mud are changed depending on operating conditions, varying the final concentration of viscous fluid between 1.0 lb/bbl (3.8 kg/m.sup.3) and the maximum concentration of Pad mud prepared on the surface, if there are pumpability conditions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention will be described in more detail below, with reference to the attached figures which, in a schematic way and not limiting the inventive scope, represent examples of its embodiment. In the drawings, there are:

[0033] FIG. 1 illustrating a static immersion of a carnallite sample in (a) seawater and (b) brine saturated in NaCl. Exposure time—15 minutes;

[0034] FIG. 2 illustrating a static carnallite dissolution test, exposure interval 12 hours;

[0035] FIG. 3 illustrating historical data on the average diameter of the saline section for the beginning of wells carried out in the same pre-salt field in the Santos Basin, where well J is an example of the application of the technique developed herein;

[0036] FIG. 4 illustrating a post job of the quality of the cementation of the surface casing, WELL J;

[0037] FIG. 5 illustrating a scheme of PAD Mud tanks and pumping. Its components are represented by: pump #1 (1), pump #2 (2), pump #3 (3), pre-load pump #1 (4), pre-load pump #2 (5), pre-load pump #3 (6), Standpipe manifold (7), mixture of Pad Mud with seawater (8), Standpipe (9), Top Drive (10), drill string (11), Tank #1 (12), Tank #2 (13), Tank #3 (14), Tank #4 (15), Tank #5 (16), Tank #6 (17), Tank #7 (18), Tank #8 (19), Tank #9 (20), Tank #10 (21), Tank #11 (22), Tank #12 (23), Slug Tank #1 (24), Slug Tank #2 (25), Chemical Treatment Tank #1 (26), Chemical Treatment Tank #2 (27), Seawater (28), Pad Mud (29), Saturated well filling fluid (30), Diluted drilling fluid (31).

DETAILED DESCRIPTION OF THE INVENTION

[0038] The use of low salinity viscous fluid in non-return drilling operations in the presence of predominantly saline formation according to the present invention comprises in a sequence below that describes a model of Pump and Dump operation, including preparation step and the operation itself. In this example, the objective is to obtain a fluid with a minimum concentration of 1.0 lb/bbl (3.8 kg/m.sup.3) of viscosifier (in this case, xanthan gum) prior to pumping the same into the well. The term PAD Mud is only to identify the fluid during the Pump and Dump operation. In addition to xanthan gum, other viscosifying polymers can be used: guar gum, carboxy-methyl-cellulose, etc.

Preparation:

[0039] Phase drilling requires the preparation of large volumes of fluids (filling fluids, viscous plugs, in addition to the PAD Mud) and, therefore, it is necessary to provide an operational team for the manufacture of fluid and alignment of transfer and suction lines.

[0040] It is observed that, in some drilling rigs, it is necessary to use a tank dedicated to pumping seawater, which restricts the volumetry of other fluids.

[0041] The PAD Mud is manufactured in the typical concentration range of 3.0 to 6.0 lb/bbl (11.4 to 22.8 kg/m.sup.3) of xanthan gum for dilution with seawater during pumping. PAD Mud can be manufactured in xanthan gum concentrations greater than 6.0 lb/bbl (22.8 kg/m.sup.3) (it is not limited, higher concentrations are even desired), as long as the final viscosity allows for pumpability. Other additives can be added to obtain other properties, for example: filter reducer, bactericidal, alkalizing agent and salt.

[0042] If a concentration greater than 6.0 lb/bbl (22.8 kg/m.sup.3) is used, the pumping flow rate of PAD Mud and seawater must be adjusted so that the output in the standpipe manifold is at the final concentration of viscosifier in the mixture a minimum of 1.0 lb/bbl (3.8 kg/m.sup.3).

[0043] During formation drilling prior to the saline layer (post-salt), the concentrated PAD Mud is pumped in the form of undiluted cleaning plugs. The volume spent during the post-salt drilling must be promptly replaced so that, when starting the Pump and Dump, the rig has the maximum volume of PAD Mud available.

[0044] Part of the concentrated PAD Mud must remain stored in surface tanks and another part in reserve tanks, or even brine (the use must be negotiated with the drilling rig contractor, as its use is restricted, in addition to evaluating issues of pumpability and agitation of the tanks). It is desirable to treat all the PAD mud that is stored prior to the operation with a bactericide, to avoid an excessive time of polymeric fluid made with seawater and standing still in the brine or reserve tanks, it is recommended to recirculate the PAD Mud to the surface tanks periodically, preemptively treating the returned fluid with glutaraldehyde to avoid degradation of the fluid and the release of hydrogen sulfide into the environment (H.sub.2S). pH should be maintained in the range of 9.0 to 10.5 with an alkalizing agent.

[0045] Alkalizing agents that remain suspended in the PAD Mud are avoided, if the tank does not have agitation.

[0046] Before starting the phase drilling, check with the rig team the alignment of the brine tanks with PAD Mud to feed the surface tanks.

[0047] The alignment of the PAD Mud and seawater suction lines is checked, as well as the alignment on the standpipe manifold for the mixture of concentrated PAD Mud and seawater. A suggested scheme for the fluids in the rig tanks before the start of the Pump and Dump can be seen in FIG. 5.

Phase Drilling:

[0048] Drill with seawater (high flow rate, for example 1,000 to 1,200 gpm) (3.7854 to 4.5425 m.sup.3/min), and pump 80 to 120 bbl (9.539 to 14.309 m.sup.3) of viscous fluid or PAD Mud 1.5 to 3.0 lb/bbl (5.7 to 11.4 kg/m.sup.3) every 3 drilled sections. Evaluate the cleanliness and adjust the frequency of plugs with the drilling parameters.

[0049] Proceed with drilling with seawater and displacement of periodic PAD Mud plugs to the top of the saline formation.

Pump and Dump:

[0050] After identifying the change in the formation of anhydrite (common at the top of the saline formation) to halite (indication given by the sudden increase in the penetration rate), the Pump and Dump operation begins, reducing the flow rate to the operating minimum of the drill string directional tool for data acquisition of LWD/MWD (LWD=Logging While Drilling and MWD=Measurement While Drilling). Only start the Pump and Dump when the drilling is already in the halite.

[0051] If the minimum flow rate is not possible due to data transmission and tool operation, it is recommended to use the lowest possible flow rate at which the tool will work. It is worth remembering that the lower the flow rate, the greater the autonomy of PAD Mud and the lesser the dissolution of the saline walls of the well.

[0052] Once the formation of halite is confirmed by the sudden increase in the penetration rate, the Pump and Dump is started by reducing the seawater flow rate to half of the expected total flow rate and adding the second pump with the same flow rate of concentrated PAD Mud.

[0053] The total drilling flow rate is maintained, mixing PAD Mud and seawater, so that the final mixture after the standpipe manifold contains at least 1.0 lb/bbl (3.8 kg/m.sup.3) of viscosifier.

[0054] If difficulties are observed to pump the PAD Mud with only one pump, more than one pump can be used to divide the flow destined for the PAD Mud in order to avoid cavitation problems (if the fluid is too viscous). Pay attention to the adjustment of the mud pumps so that the final concentration in the drilling flow, after mixing the PAD Mud with the seawater flow in the standpipe manifold, is at least 1.0 lb/bbl (3.8 kg/m.sup.3).

[0055] After the start of the Pump and Dump, when the first tanks of PAD Mud on the surface run out, there is immediately started the preparation (or dilution) of the filling fluid complement for running the casing down, and for transfers during the withdrawing of the bit.

[0056] If anhydrite intercalations occur with a reduction in the drilling rate, it is recommended to reduce the PAD Mud flow rate and increase the seawater flow rate so that the fluid concentration after dilution in the standpipe manifold is lower during the section with low drilling rate. If the advance is too low, evaluate the possibility of using a lower final concentration of viscosifier (by adjusting the PAD Mud and seawater flow rates) to increase the autonomy of this fluid.

[0057] At the end of the drilling phase, pump the saturated filling fluid into the bottom to occupy the entire evaporite section.

[0058] The string is withdrawn to the top of the evaporite section and filling fluid is pumped to occupy the remainder of the annulus to the seabed.

Examples

[0059] The following examples are presented in order to more fully illustrate the nature of the present invention and the way to practice the same, without, however, being considered as limiting its content.

[0060] In a static immersion test of a carnallite pill in seawater and in a fluid saturated in NaCl, for an exposure of only 15 minutes, it is possible to verify a sensitive dissolution both in seawater (FIG. 1a) (more pronounced, 82%) and in NaCl brine (FIG. 1b) (27%). It is important to note that in (b) two effects occur: the preservation of halite and the recrystallization of NaCl salts on the pill.

[0061] The recrystallization of NaCl salts on the pill is a demonstration of the displacement of ionic equilibrium, in which, as the diffusion of carnallite into the immersion fluid occurs, there is a displacement of sodium chloride due to the introduction of ions in common, forcing its precipitation.

[0062] In a study carried out to investigate the equilibrium state where the dissolution of a carnallite sample would no longer be evident, focusing on the first 12 hours, analyzing only the variation in salinity and the effect of viscosity, it is possible to notice that the curve behavior (seawater+2.0 lb/bbl (7.61 kg/m.sup.3) xanthan gum) resembles a saline solution with 360,000 mg/L NaCl. Under static conditions, the best results are obtained when viscosity and salinity effects are associated (FIG. 2).

[0063] To show the predominant effects, a field test was carried out using viscous seawater to drill the last 120 m of phase salt, without using a dedicated vessel to supply the fluid. In the investigated case, a 50% v/v mixture of seawater with a 50% v/v polymeric Water-Based Drilling Fluid (FPBA) viscosified with xanthan gum, manufactured with additives and seawater, in which the xanthan gum concentration was 3 lb/bbl (11.41 kg/m.sup.3). The planned procedure was to manufacture a volume sufficient for 12 hours of drilling, a time based on wells or related drilling. Therefore, resources were previously mobilized and 5,500 bbl (655.8 m.sup.3) were manufactured. It was agreed that the drilling flow rate for the saline section would be between 600 and 650 gpm (2.27 and 2.46 m.sup.3/min). Some of the important results were: [0064] The global operating time was approximately 40% lower than the reference wells, due to the high performance of the bit used in the phase; [0065] The back analysis of the well diameter indicated an improvement in the condition of the well for successful cementation, with an average diameter of 31.2″ (79.248 cm) in the saline section against an average of 37.6″ (95.504 cm) in the historical series of the field (wells where phase 2 was all drilled with seawater). FIG. 3 presents a graph with the historical data used for the analysis of the wells in the field and the used well J that refers to the field test. This is a 43% reduction in the volume of dissolved salt (compared to the historical average), which contributes to improving the conditions for cementation; [0066] An analysis of the quality of the cementation was carried out with the data collected during the operation with a commercial removal simulator. FIG. 4 illustrates, on the left, the slurry filling in the annulus without the presence of channeling. On the right, it illustrates the concentration of fluids at a depth 140 m above the shoe, where a high concentration of cement slurry can still be observed.

[0067] It is concluded that, with the present invention, it is possible to improve the quality of the well with a significant reduction in salt dissolution at optimized costs with the objective of reducing a phase simultaneously with the Well Containment Analysis (WCA). Thus, the present invention presents advantages such as lower operating costs, reduction in the number of phases, application in a third phase without return (deep post-salt cases) with expected gain of up to 5 days (reduction of well construction time), in addition to employing a low salinity fluid.

[0068] It should be noted that, although the present invention has been described in relation to the attached drawings, it may undergo modifications and adaptations by technicians skilled on the subject, depending on the specific situation, but provided that it is within the inventive scope defined herein.