Pak Prawoto, Mungkin tulisan yang saya sarikan ini bisa membantu untuk menjawab pertanyaan no.2
Konversi unit log *Gamma Ray* dari CPS ke GAPI (Gamma API) atau lebih sering disebut API, dilakukan berdasarkan hasil kalibrasi tool standar pada fasilitas tes API (*American Petroleum Institute*) yang berada di Houston. Satu GAPI di definisikan sebagai 1/200 perbedaan defleksi log (atau pengukuran *count rates*) antara zona dengan radioaktifitas tinggi dan zona yang memiliki redioaktifitas sangat rendah pada tes pit API tersebut. Untuk tool standar yang di log pada tes pit ini, *sensitivity factor* (atau *gain*) di hitung dengan persamaan *Sensitivity = 200 GAPI / (High Count Rate – Low Count Rate)* Sensitivitas merupakan faktor konversi untuk mengubah satuan hasil pengukuran (*count per second*) menjadi GAPI. Setelah harga sensitivitas dari tool standar diketahui, maka renspon tool tersebut pada setiap well dapat dinyatakan sebagai: *GAPI = Sensitivity x Raw Count Rate* Pada prakteknya, akan sulit bagi service companies untuk mengkalibrasi setiap tool *gamma ray* mereka di tes pit API. Untuk itu mereka menggunakan *calibrator*, yang dibuat berdasarkan respon tool standar dan sensitivitas yang telah diketahui, untuk melakukan kalibrasi terhadap setiap tool gamma ray yang mereka gunakan. Salam, Eril S. Lanin On Mon, Sep 15, 2008 at 8:36 AM, prawoto syuhada <[EMAIL PROTECTED]>wrote: > Pak Nyoto, > terima kasih pak file Pdf-nya > > Saya sendiri juga baru tahu kalau log GR jaman dulu satuaannya bukan API > tetapi masih CPS... > > Praw > > > > > > ----- Original Message ---- > From: nyoto - ke-el <[EMAIL PROTECTED]> > To: [email protected] > Sent: Thursday, September 11, 2008 12:13:37 PM > Subject: Re: [iagi-net-l] Tanya mengenai unit Log > > Pak Prawoto, > > Bisa coba dilihat di link dibawah ini (dari Schlumberger Log Principles > Textbook) : > > http://www.slb.com/content/services/resources/books/lipa.asp?rky=552&; > > Disitu ada file2 pdf-nya yg bisa di-copy langsung, didalam buku2 tsb ada > teori mengenai log GR & Density serta log2 lainnya, bagaimana cara > pengukurannya & kalibrasinya ke angka2 porosity dll. > > Saya akan coba attach-kan salah satu textbooknya yg ada teori log GR & > density, tapi ke japri aja ya, soalnya filenya besar2. > > Dibawah (warna biru) ini saya coba juga copykan sedikit "teori"nya. > > Semoga membantu. > > > > wass, > > nyoto > > > > > > > > DENSITY LOGS > > Density logs arc primarily used as porosity logs. Other > > uses include identification of minerals in evaporite > > deposits, detection of gas, determination of hydrocarbon > > density, evaluation of shaly sands and complex > > lithologies, determinations of oil-shale yield, calculation > > of overburden pressure and rock mechanical properties. > > Principle > > A radioactive source, applied to the borehole wall in a > > shielded sidewall skid, emits medium-energy gamma rays > > into the formations. These gamma rays may be thought > > of as high-velocity particles that collide with the electrons > > in the formation. At each collision a gamma ray loses > > some, but not all, of its energy to the electron, and then > > continues with diminished energy. This type of interaction > > is known as Compton scattering. The scattered gamma > > rays reaching the detector, at a fixed distance from > > the source, are counted as an indication of formation > > density. > > The number of Compton-scattering collisions is related > > directly to the number of electrons in the formation. Consequently, > > the response of the density tool is determined > > essentially by the electron density (number of electrons > > per cubic centimeter) of the formation. Electron density > > is related to the true bulk density, eb, which, in turn, > > depends on the density of the rock matrix material, the > > formation porosity, and the density of the fluids filling > > the pores. > > Equipment > > To minimize the influence of the mud column, the skidmounted > > source and detector are shielded. The openings > > of the shields are applied against the wall of the borehole > > by an eccentering arm. The force exerted by the arm, and > > the plow-shaped design of the skid, allow it to cut through > > soft mudcakes. Any mudcake or mud remaining between > > the tool and the formation is "seen" as part of the formation > > and must be accounted for. > > A correction is needed when the contact between the > > skid and the formations is not perfect (mudcake or irregularities > > in the borehole wall). In unfavorable cases > > this correction can be fairly large. If only one detector > > is used, the correction is not easy to determine because > > it depends on the thickness, the weight, and even the composition > > of the mudcake or mud interposed between the > > skid and the formations. > > In the FDC* compensated formation density tool, two > > detectors of differing spacing and depth of investigation > > are used, as shown on Fig. 5-12. The chart of Fig. S-13 > > is a plot of long-spacing versus short-spacing count rates. > > Points for a given value of eb and various mudcake conditions > > fall on or very close to an average curve. Using > > Fig. B-12-Schematic drawing of the dual spacing Formation > > Density Logging Device (FDC). (Ref. 16) > > Fig. 5-13-"Spine-and-ribs" plot, showing response of FDC > > counting rates to mudcake. (Ref. 16) > > these average curves it is possible to enter the chart with > > the two count rates and determine the corrected eb from > > the plot without any explicit measurement of mudcake > > density or thickness. This measurement technique is referred > > to as "spine and ribs." > > The correction is made automatically and the corrected > > ~b curve and Ae (the correction made) are recorded > > directly on the log (Fig. S-14). > > The distance between the face of the skid and the extremity > > of the eccentering arm is recorded as a caliper > > log, which helps to assess the quality of contact between > > the skid and the formation. > > > > > > > > > > > 2008/9/11 prawoto syuhada <[EMAIL PROTECTED]> > > > > > > > Bapak dan ibu sekalian, ada pertanyaan dari seorang mahasiswa, apakah ada > > yang bisa bantu???? > > Pertanyaan lengkapnya: > > > > Perkenalkan nama saya Hidayat Saputra > > Saya seorang mahasiswa Teknik Geofisika yang sedang melaksanakan Tugas > > Akhir (TA). > > Saya ingin bertanya pak..... > > > > 1. Bagaimana prinsip dasar pengukuran log GR dan log density > > sehingga bisa di dapat satuan Counts Per Second (CPS). atau API dan > > Gramm/Cc > > 2. Bagaimana cara mengkonversi log Gamma Ray dari CPS ke pada API. > > 3. Bagaimana cara mengkonversi log Density dari Counts Per Second (CPS) > ke > > Gram/ CC > > > > Adakah Rumus yang bisa digunakan untuk mengkonversi satuan > > tersebut. Bagaimana cara perhitungannya? Adakah buku atau paper yang > > dapat saya baca untuk penjelasan diatas? > > Bantuan dan penjelasan dari Bapak sangat saya harapkan > > > > Atas perhatiannya saya ucapkan terima kasih. > > Jawabannya di tunggu ya...... > > > > Hormat saya > > Hidayat S. > > > > > > > > > > >
