METHOD OF INCREASING LONGITUDINAL STABILITY OF WELL STRUCTURE IN PERMAFROST Russian patent published in 2008 - IPC E21B36/00 

FIELD: oil and gas industry.

SUBSTANCE: invention refers to oil and gas industry and can be used particularly at making decision on construction process, on wells structure and on measures to be taken to increase their longitudinal stability in thawing out with subsidence, and cavernous permafrost rocks (PFR). Method includes well boring, control over heat mode of construction and operation of well, lowering of thermometric pipes (TMP) beyond the outer column into a well head zone while the annular space (AS) of the latter is subject to cementing, then lowering into TMP seasonally operating cooling devices (SOCD) to reduce intensity of PFR thawing out and provide support to the well structure onto surrounding rocks maintained in frozen condition. Then well caliper survey is performed, an upper cavernous with subsidence interval of PFR with length L_cl is determined, and critical length (L_cr) of maintaining longitudinal stability of a well structure is calculated. When boring, a borehole adjacent to the surface is reamed with a reamer of diameter D_r taking into consideration cavernosity of a borehole up to depth L_cl; also embedding of a borehole with reaming into rocks, stable at thawing out and deposited below depth L_cl,_{is performed. Diameter of the reamer Dr is determined from a reduced mathematical expression in a formula. Cementing of the well borehole is carried out, and by means of TMP control over quality of cementing, stabilisation of the well and assessment of longitudinal stability of the structure is performed. There are revealed cryolite zones with subsidence, cavernous PFR, and caverns not filled with cement beyond the outer column, overlapping PFR. There are determined dimensions of gaps near surface and at depth around the well including their diameters and depth, also of gaps between the cement ring around the outer column and rock in AS. On base of obtained measurements and on radii of thawing out of PFR there are determined the summary lengths of upper and lower subsidence zones, lengths (extensions) of caverns, adjacent to them, and cavernous intervals; on reduced expressions the summary volumes of gaps are determined. Further there are determined maximum continuous intervals, lengths Lg of not filled gaps, caverns, not filled with cement, in PFR with subsidence, cavernous PFR, in the upper adjacent to the surface interval of cryolite zone, which can be filled, and in lower interval, where filling has not reached. At that Lg must not exceed the critical length Lcr of longitudinal stability of the well structure (Lcr>Lg) for these intervals at loads, effecting onto the well. To increase rigidity of the well structure and value of Lcr, and also to increase longitudinal stability of the structure at well construction its borehole is reamed in cavernous PFR with subsidence within the dimensions of revealed caverns, and the thickness of the cement ring is increased beyond the outer column taking into consideration loads from Pcr1 to Pcr2 (Pcr2>Pcr1), growing during well operation; these loads effect on the well structure at lowered pressure onto its wellhead to the value of δc2 according to the reduced expression. At that, during operation TMP is lowered from the wellhead into a gap created around the wellhead at a maximum depth of their passing into AS, after what the gap is filled with a filling material, for example, with sand and/or soil including into it heat insulating materials and additives, lining the well structure in surrounding rocks. When the gap has been filled, the temperature in TMP is measured during operation of the well; on base of temperature measurements thawing out of PFR in AS is evaluated. The dynamics of gaps development at thawing out of rocks around the well after filling the gap is evaluated on TMP pipes movements. At winter time there are lowered SOCD into TMP pipes installed in a gap; SOCD serve for freezing of thawed out PFR and for reducing their thawing out intensity.}

EFFECT: upgraded reliability of well structure.

1 dwg, 1 tbl