Recovery of heavy oils by steam extraction

Claim 1

IN A METHOD OF RECOVERING, FROM AN UNDERGROUND STRATUM IN WHICH IT OCCURS, HEAVY OIL WHICH IS SUBSTANTIALLY NONFLOWABLE BY THE APPLICATION OF DRIVING-FLUID PRESSURE THERETO AND WHICH RETAINS SUBSTANTIAL VISCOSITY AT ELEVATED TEMPERATURES, SAID STRATUM BEING SUBSTANTIALLY COMPLETELY PENETRATED BY A WELL EXTENDING FROM THE GROUND SURFACE, WHICH METHOD COMPRISES THE STEPS OF INJECTING SUBSTANTIALLY ONLY STEAM AT AN ELEVATED TEMPERATURE AND PRESSURE INTO SAID WELL AND THENCE INTO SAID STRATUM TO HEAT BY CONDENSATION SUBSTANTIALLY THE ENTIRE STRATUM FACE EXPOSED IN SAID WELL AND REDUCE THE VISCOSITY OF THE OIL AT SAID FACE, WHEREBY HEATED OIL AND STEAM CONDENSATE FLOW DOWNWARDLY BY GRAVITY DRAINAGE TOWARD THE BOTTOM OF SAID WELL AND CONTINUOUSLY EXPOSE UNHEATED OIL AND FORMATION BEHIND SAID FACE, AND WITHDRAWING SAID HEATED OIL AND CONDENSATE FROM NEAR THE BOTTOM OF SAID WELL AT SUBSTANTIALLY THE RATE THEY COLLECT THERE WHILE HOLDING SUFFICIENT BACK PRESSURE ON SAID WELL TO MAINTAIN SAID PRESSURE AND TEMPERATURE AT THEIR ELEVATED VALUES WITHIN THE HEATED VOLUME OF SAID STRATUM, WHEREBY A ZONE OF MELTING OIL AND GRAVITY DRAINAGE OF HEATED OIL AND STEAM CONDENSATE PROPAGATES RADIALLY OUTWARDLY FROM SAID WELL THROUGH AT LEAST THE UPPER PART OF SAID STRATUM, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF DISCONTINUING SAID STEAM-INJECTING, WITHDRAWING AND BACK-PRESSURE-HOLDING STEPS AT A TIME WHEN THERE IS AN ACCUMULATION OF HEATED HEAVY OIL AND STEAM CONDENSATE OF SUBSTANTIAL SIZE WITHIN SAID STRATUM, AND RAPIDLY WITHDRAWING FLUIDS FROM NEAR THE BOTTOM OF SAID WELL TO REDUCE THE PRESSURE AT SAID WELL BORE TO A LOW VALUE AND THEREBY ESTABLISH THROUGHOUT SAID HEATED VOLUME A SUBSTANTIAL PRESSURE GRADIENT TO AID THE FORCE OF GRAVITY DRAINAGE IN MOVING SAID ACCUMULATION TOWARD SAID WELL BORE.