As fluid escapes from the pores of clayey rocks, the rocks undergo compaction and porosity decreases. However, if the escape of fluid is inhibited because of their low permeability, the compaction may not occur and a high porosity or high fluid pressure situation will result. Such a low permeable mudstone is probably good for the barrier against water expulsion. Therefore, if there exists the mudstone of this kind, some of the compaction current could move downward in the level below this low permeable zone, and move upward in the level above this zone. The volume of the compaction current is estimated from the porosity decreases measured with subsurface data such as Gamma Gamma log and Induction log at Shiunji gas field, Niigata Prefecture. The result of the estimation is discussed in connection with the possible hydrocarbon accumulation at Shiunji.
Experiments on the balling up in drilling were carried out using an apparatus simulating a bore hole with a drill pipe. Results obtained are as follows: (1) the low upward velocity of cutting and the eccentricity of a drill pipe in a bore hole, especially in high angle directional drilling cause the balling up around a drill pipe, and (2) the high viscosity of liquid prevens to produce the balling up.
By the nse of a 11/4 in. 2-cone micro-bit and 33/4 in. soft formation type jet bit, experiments on the balling-up were carried out. Bit balling-up and chip remaining at a bottom of a bore hole are main factors and decrease penetration rate and that increase torpue for rotation. They cause a low efficiency of the drilling.
In Kubiki district many high angle directional wells have been drilled to develop off-shore gas fields. In drilling a well with defflection angle of more than 45 degree, a severe balling-up around a bit and drillpipes occurs occasionally and it causes a low drilling speed. To solve this problem quantity of cuttings per unit length were measured throughout a drilling of a wall. As a result of analyzing these data, were obtained the conclusion that factors such as circulating rate of drilling mud, physical properties of drilling mud and etc. relate directly with the balling-up in directional drilling.
Approximately linear or exponential flow rate decline is often reported at the conventional DST or Flow Test. At the present study, a new practical method based on theoretical considerations is brought forward for pressure build-up analysis in such cases. Its characteristic feature exists in plotting the observed pressure data vs the appropriate functions different from that of logarithmic type as in the conventional methods, which are derived, taking the flow period, shut-in period and decline rate into considerations, and the numerical values of which can be easily known on the prepared charts. This new method provides with the simultaneous evaluation of prevailing reservoir pressure, transmissibility and flow decline rate, even though the last is previously unkown, and it seems to the author that better results can be obtained by the new method than by other conventional ones. The evaluation of skin factor is also treated at the present study. As a matter of course, the skin factor formula for variable rate case differs from the conventional one for constant rate case and the accuracy of results by the latter is theoretically discussed. It is considered that the modification for the skin factor formula has its practical meaning especially at DST, because the final flow rate can't generally be known. The full contents will appear in instalments.