How are you? i am yousuf, I am following your answers from the long time. I have been guided by you many times before also, and i am very grateful for this.
Sir This time I come up with some confusion regarding formation tops, what should be the steps to mark the formation tops else than Gamma ray (just on the base of Physical Appearence), Most of the well site geologist mark the formation tops or try to catch the tops by matching R.O.P from the offsets wells , do you think it is the good idea because sir ( sometime there is the difference of choke and cheese in drilling parameters of offset and current wells) and you know that the driller are not reliable.
and second one is that what should be the criteria for casing point, i have seen that sometime they have chosen the shale lithology having 50 feet of thickness under the limestone.. thanks
To correlate with GR with nearest offset well is a reliable control point and I would advise you to use it but you have to consider the well trajectory and dips of the beds that may confuse you.
For example, if a well penetrate high dips thicknesses of the beds will be greater compare to the dips with low magnitude or near horizontal beds. In similar fashion, the well trajectory also play a role (high inclination of well pathway in horizontal beds may read abnormally high thickness).
ROP is also useful method to correlate with nearby well on mud logs. Although there are several factors that change the ROP values. For example, if driller use a higher Weight on bit (WOB) and Rotation per minute (RPM) in one well whereas other well was drilled with considerably lower WOB and RPMs but you do not necessarily need to match the exact ROP values (say 10 ft per hour to 10 ft per hour) but you have to identify and establish the trend of the increasing and decreasing ROP (also check relative drilling break and reverse break). Check the lithology as well against each ROP you are trying to correlate as not all sandstone are drilled quickly nor all shales drilled with relative ease.
Casing seat is usually placed at the impermeable and un-fractured rock where cement losses should be minimum while circulating cement.
Hope, this answer may be helpful.